To observe the developmental process and characteristics of Trichinella spiralis muscle larvae in mice, 28 female Kunming mice were orally infected with 100 T. spiralis muscle larvae and each two mice were euthanized at 8, 9, 10, 11, 13, 17, 20, 23, 27, 29, 50, 60, 240 and 400 days post infection. The diaphragm and partial abdominal muscles were collected, sectioned and stained with alum carmine acetate staining. The muscle larvae, larval capsules were observed under microscope. The muscle larvae were found in the diaphragm as early as 10 days post infection (10 d) and in abdominal muscles at 13 d, mostly with straight shape. The larvae were shaped in curve, circle, folded and larval capsules were formed at 17 d. The capsular bags were well developed, the stripes were seen in the wall of the capsule, and the capsule structure was divided into two layers without clear boundary at 29 d. The larvae twisted into a mass, and the structure inside the capsule began to appear transparent at 60 d. The capsule started to degenerate and the wall of the capsule began to appear transparent at 400 d. The average size of muscle larvae was (131.7 ± 21.4) μm × (10.4 ± 2.5) μm at 10 d, (435.5 ± 201.5) μm × (22.8 ± 5.2) μm at 17 d, (928.4 ± 188.5) μm × (35.0 ± 5.2) μm at 29 d and (1044.4 ± 86.7) μm × (38.3 ± 3.0) μm at 60 d, with significant difference in size between each developmental time points (P ﹤ 0.05 or P ﹤ 0.01). According to the experimental results, larval and capsular development can be roughly divided into four stages: pre-encapsulated larvae, larvae with capsular development, larvae with mature capsular bag, and larvae with degraded capsular bag. The morphological structure of larvae and capsules is closely related to the time of infection.