Objective To investigate the effects of Schistosoma japonicum infection (21 days after infection) on ovalbumin (OVA)-induced allergic airway inflammation in mice. Methods Twenty female BALB/c mice were randomly divided into the healthy control group, infection group (INF), OVA group and infection+OVA group (INF+OVA), 5 mice each. Mice of the INF and INF+OVA groups were infected with 15 ± 2 S. japonicum cercariae by abdomen patches. Mice in the INF+OVA group and OVA group were intraperitoneally injected with 10 μg of OVA plus 2 mg of aluminum adjuvant to sensitize on day 21 and 35 after infection, and were given with aerosolized 1% OVA in PBS/mouse to challenge on day 42-46 after infection, whereas the mice of healthy control and infection group were challenged with PBS aerosol. Mice tail vein blood was collected on day 35 and 42 (sensitizing phase), and day 49 (challenging phase) for determination of serum IgE and OVA-specific IgE by ELISA. All mice were sacrificed on day 49 after infection, and lung and spleen tissues were collected. A part of the lung tissue was used to prepare pathological paraffin sections stained by hematoxylin eosin (HE) and periodic acid Schiff (PAS), to observe morphological changes in lung tissue vessel, inflammation peri-bronchi (inflammation score), and in hyperplasia of epithelial cells (hyperplasia score); the other part of lung and spleen tissues was used to prepare single cell suspensions for assaying proportions of CD4⁺CD25 ⁺Foxp3⁺ Tregs by flow cytometry. Results ELISA showed that the A₄₅₀ readings of serum IgE in the INF group on day 35, 42 and 49 were 0.291 ± 0.037, 0.388 ± 0.038 and 0.472 ± 0.053, respectively, which were significantly higher than those in healthy control (0.032 ± 0.018, 0.050 ± 0.016 and 0.043 ± 0.026) (all P < 0.01); the A₄₅₀ readings of IgE in the OVA group on day 35, 42 and 49 were 0.097 ± 0.065, 0.164 ± 0.083 and 0.274 ± 0.07, respectively, of which the readings on day 42 and 49 were significantly higher than those in healthy control (P < 0.01); the A₄₅₀ readings of IgE in the INF+OVA group on those days were 0.305 ± 0.054, 0.359 ± 0.037 and 0.454 ± 0.017, respectively, all significantly increased compared to the OVA group (P < 0.01). On day 35, 42 and 49 after infection, the A₄₅₀ values of OVA-specific IgE in the OVA group were 0.115 ± 0.021, 0.078 ± 0.014 and 0.245 ± 0.040, respectively, all significantly increased compared to the control group (0.081 ± 0.007, 0.054 ± 0.008, and 0.079 ± 0.008) (P < 0.01, 0.05, 0.05). However, those in the OVA+INF group (0.034 ± 0.009, 0.038 ± 0.016 and 0.203 ± 0.017) were all significantly lower than those in the OVA group (P < 0.01, 0.01, 0.05). Pathological examinations on day 49 after infection showed that the lung tissue inflammation scores on inflammatory cell infiltration peri-bronchi and blood vessels in the healthy control group, INF group, OVA group and INF+OVA group were 1.33 ± 0.47, 2.33 ± 0.27, 3.5 ± 0.19 and 2.87 ± 0.45, respectively, of them the inflammation score in the INF and OVA groups were both significantly increased, compared to the healthy control group (P < 0.01), while the score in the INF+OVA group was significantly reduced (P < 0.05). The hyperplasia scores in the healthy control group, INF group, OVA group and INF+OVA group were 0, 0, 2.12 ± 0.80 and 1.72 ± 0.55, respectively. The hyperplasia score in the OVA group was significantly increased compared to the healthy control group (P < 0.05), whereas the difference between the INF+OVA group and the OVA group was insignificant (P > 0.05). The assay results of flow cytometry on day 49 after infection showed that the proportions of Tregs in spleen and lung tissues were (16.24 ± 3.06)% and (10.19 ± 0.01)% in the healthy control group, (27.23 ± 5.62)% and (13.05 ± 1.10)% in the INF group, (17.22 ± 1.43)% and (12.34 ± 2.25)% in the OVA group, and (27.96 ± 1.80)% and (15.04 ± 1.41)% in the INF+OVA group, respectively. The lung and spleen Treg proportions in the INF group were significantly up-regulated compared to the healthy control group (both P < 0.05), while there was no significant difference between the OVA and control groups (P > 0.05). The spleen tissue Treg proportion showed a significant increase in the INF+OVA group compared to the OVA group (P < 0.01), while the lung Treg proportion showed no significant change (P > 0.05). Conclusion S. japonicum infection (21 days after infection) showed rsignificant regulatory effect on the OVA-induced allergic airway inflammation, reducing the serum OVA-specific IgE level at the sensitization and challenge phases of allergic inflammation, suppressing peri-bronchi and peri-vascular inflammatory cell infiltration induced by OVA, and leading to elevate Treg proportion in the spleen tissue.