Objective To explore the effect of praziquantel treatment on the number and function of splenic macrophages in mice infected with Schistosoma japonicum. Methods Thirty-five female C57BL/6J mice were randomly divided into healthy control group, uninfected plus praziquantel-treatment group, infected group, infected plus praziquantel-treatment group, solvent control group, adoptive transfer group and adoptive transfer treatment group (5 mice each). Mice in the latter five groups were each infected with 14 ± 2 cercariae of S. japonicum through abdominal skin. Ten weeks after infection, the mice in the solvent control group were injected with sterile saline through tail vein, and the mice in the adoptive transfer group and the adoptive transfer treatment group were injected with (1-2) × 10⁶ green fluorescent protein (GFP)⁺Ly-6C⁺ monocytes through tail vein. The mice in the uninfected plus praziquantel-treatment group, the infected plus praziquantel-treatment group and the adoptive transfer treatment group were given praziquantel treatment (300 mg/kg every 12 h) for consecutive 28 days. The mice in healthy control group, infected group, solvent control group and adoptive transfer group were gavaged with 1% carboxymethyl cellulose solution every 12 hours for consecutive 28 days. Then spleen tissues from each group of mice were taken to evaluate the effect of praziquantel treatment on the spleen tissue damage by HE staining. To evaluate the effect of praziquantel on the proliferation of macrophages, the number of Ly-6C⁺ macrophages and the expression of Ki-67 within Ly-6C⁺ macrophages in the spleen were assessed by flow cytometry. Real-time fluorescence quantitative PCR (qRT-PCR) was performed to detect the expression of inflammatory factors TNF-α and IL-1β in mouse splenic macrophages. Flow cytometry was used to calculate the proportions of GFP⁺Ly-6C⁺ macrophages in the spleen in order to evaluate the effect of praziquantel on their recruitment. Adherent macrophages from spleens of mice 10 weeks post-infection were used for in vitro experiment. The macrophage culture was treated with 20 μg/ml praziquantel or dimethyl sulfoxide for 24 h, consequently, flow cytometry was used to examine the proportion of Ly-6C⁺ macrophages in the collected wall-adherent cells of the two groups. Results Visual inspection and HE staining showed clear internal structures of spleen parenchyma of mice in the healthy control group and uninfected plus praziquantel-treatment group, with white and red pulps in clear structure, splenocytes arranged regularly, no sign of pathological damage to the spleen. Splenomegaly was seen in mice of the infected group, with disordered histological structure, unclear distribution of red and white pulps, partial congestion of the spleen sinus, and irregular arrangement of cells. In the mice of the infected plus praziquantel-treatment group, the splenomegaly was relieved, the parenchymal structure repaired, the red and white pulps reformed, and the spleen cells distributed regularly. The flow cytometry showed that no GFP-positive macrophages were found in the spleens of mice in the solvent control group, adoptive transfer group and adoptive transfer treatment group. The total proportions of splenic macrophages in the healthy control group, uninfected plus praziquantel-treatment group, infected group, and infected plus praziquantel-treatment group were (6.1 ± 1.4)%, (6.0 ± 0.6)%, (21.3 ± 2.3)%, and (7.8 ± 1.6)%, respectively; the relative proportions of Ly-6C⁺ macrophages were (27.3 ± 2.4)%, (26.6 ± 1.5)%, (57.3 ± 5.2)%, and (25.7 ± 2.8)%, respectively; the absolute numbers of Ly-6C⁺ macrophages were (0.3 ± 0.1) × 10⁶, (0.3 ± 0.1) × 10⁶, (23.7 ± 4.8) × 10⁶, and (0.4 ± 0.1) × 10⁶, respectively; and the proportions of Ly-6C⁺ macrophages positive for intracellular Ki-67 were (39.8 ± 7.6)%, (51.3 ± 2.3)%, (64.3 ± 11.5)%, and (40.4 ± 2.9)%, respectively. These indicators were all significantly higher in the infected group than in the healthy control group (P < 0.05 or 0.01), and lower in the infected plus praziquantel-treatment group than in the infected group (P < 0.01). The results of qRT-PCR showed that the relative transcription levels of TNF-α mRNA in splenic macrophages of the healthy control group, uninfected plus praziquantel-treatment group, infected group, and infected plus praziquantel-treatment group were 0.9 ± 0.1, 1.2 ± 0.01, 6.3 ± 0.7, and 0.9 ± 0.1, respectively; the relative transcription levels of IL-1β mRNA were 1.0 ± 0, 1.7 ± 0.7, 9.4 ± 0.9, and 1.4 ± 0.1, respectively; both were significantly higher in the infected group than in the healthy control group (P < 0.05 or 0.01), and significantly lower in the infected plus praziquantel-treatment group than in the infected group (P < 0.01). The flow cytometry assay of in vitro experiment showed that the percentages of Ly-6C⁺ macrophages in the control group and praziquantel treatment group were (85.4 ± 0.6)% and (82.1 ± 0.6)%, respectively with significant difference between the two(P < 0.05). Conclusion Praziquantel could inhibit the proliferation of Ly-6C⁺ macrophages in the spleen of mouse infected with S. japonicum and reduce the expression of inflammatory factors, thereby alleviating splenomegaly and splenic inflammation in mice.