Rattus losea individuals were collected from beaches and farmland of the Poyang Lake region in 2015 to 2016. A total of 175 specimens were captured, and 171 animals were autopsied to determine their reproductive condition. Position and size of testes were noted in males. For females, the reproductive tract was examined for the tubal condition of the uterus, number and size of embryos, and placental scars. Specimens were assigned to three age classes based on carcass weight:juvenile (carcass weight ≤ 36 g), sub-adult (carcass weight from 36 to 55 g), and adult (carcass weight > 55 g). Males with testes that had dropped into the scrota and females for which the uterus was tubal were considered sexually mature. Females with embryos or visible uterine scars were considered reproductive animals. The total sex ratio (F/M) was 81.36%, but it varied in different seasons and among different age classes. The proportion of males was higher than that of females, except during summer, and the female-male ratio in spring, autumn, and winter was greater than 50.00%, reaching 133.33% in winter. The female-male ratio was also more than 50% in farmland and beaches, although it was higher in farmland. Among the different age groups, the ratio was the highest in juveniles (140.00%), followed by that of adults (90.20%). The total percentage of pregnancy in captured females was 45.26%. The average number of embryos in a litter was 6.74, and the index of reproduction for all females was 1.36. Rates of pregnancy were higher in summer and autumn, with no breeding in winter. The index of reproduction was higher in spring and autumn, and exhibited an obvious bimodal characteristic, of which the major peak was in autumn. It was shown that spring and autumn were the peak breeding periods for females. The total rate of reproductive in females was 71.58%. Seasonal variation showed a single peak curve, which was the highest in summer, followed by autumn and spring, with no pregnant females in winter. With increasing age, the average number of embryos in a litter increased significantly. Litter size and the reproductive index were highest in the adult group. The main group to reproduce was the adult class. For males, the rate of testes in scrota for all males was 81.36%, and the four seasons averaged 79.64%. Almost all captured males had scrotal testes in spring, summer, and autumn, whereas the rate of scrotal testes in winter (25.93%) was lower. The size of testes in scrota of mature males changed significantly among seasons (P < 0.01), and there was a significant difference between spring and winter. These data showed that the reproductive potential of males exhibited obvious changes after spring. Reproductive traits of males were similar to those of females by season and age class. As such, it could be concluded that the rat breeds during the spring to autumn, and stops breeding in winter. The most favorable breeding seasons of this species are spring and autumn, and the adult group is the main reproductive class.