To investigate the relationship between the infectivity of encysted larvae and the developmental stage of Trichinella spiralis. Eighteen female Kunming mice were orally infected with 20 encycted larvae each mouse. On days 60, 100, 200, 300, 400 and 500 after infection, three mice in each group were sacrificed by euthanatized, and 10 flesh samples (20 cysts/flesh sample) were used to orally infect 10 female Kunming mice. On day 30 after infection, intact diaphragms were obtained from the mice, weighted, and observed under a microscope to count the cysts. The worm load per gram of diaphragmatic muscle was calculated. In the day 60 group, the walls of cysts were intact, the edge of the deep staining layer was transparent, and larvae were seen. In the day 100 group, the cyst walls were intact, transparency appeared outside the edge of the deep staining layer, and the larvae could be seen; two cysts had no cyst wall or internal structure, with faintly visible larvae. In the day 200 group, the cyst walls were intact, light traces appeared outside the edge of the deep staining layer, and the larvae could be seen; four cysts had no cyst wall or internal structure, with barely visible larvae. In the day 300 group, the cyst walls were intact, the transparent zone and trace seen, and the larvae were visible; four cysts had no cyst wall or internal structure, with faintly visible or invisible larvae. In the day 400 group, most of the cysts had intact walls, the transparent zone and trace were visible, and the larvae were visible or faintly visible; in twelve cysts devoid of wall and cystic structure, the larvae in the cysts were visible, indistinct or invisible. In the day 500 group, the cyst walls were inapparent or even disappeared, with a transparent zone at the edge of the deep staining layer, and the larvae were visible or faintly visible. In the day 60-500 infection group, the average length of diaphragm muscle worm was (1 026.6 ± 64.8), (1 041.1 ± 62.8), (1 031.5 ± 75.6), (1 047.9 ± 56.0), (1 030.5 ± 72.2), and (1 011.2 ± 95.0) μm, respectively. In the day 60-400 infection group, the mean cyst length was (371.4 ± 69.1), (309.4 ± 40.3), (311.9 ± 48.1), (299.5 ± 43.8) and (294.3 ± 23.4) μm respectively, and the difference was statistically significant between day 60 and day 100 groups (P < 0.01). In microscopic examination of all 6 experimental groups, larval cysts were found in diaphragm muscle. This suggested that with prolonged parasitism, increased number and degree of aging of larval cysts would lead to decreasing infectivity.