Brandt's vole (Lasiopodomys brandtii), a seasonal breeder, is one of major rodent pests in Inner Mongolia grasslands. In previous literature, voles born in different seasons may have different reproductive strategies, but there is still a lack of endocrine evidence. In present study, using a mark-recapture method, we successively measured the testicular descent ratios and chronic variations of fecal testosterone and cortisol levels of male voles born in different seasons, and then analyzed their seasonal changes in reproductive activity in natural enclosure. Our results showed that higher testicular descent ratio and higher testosterone levels of overwintered voles could be maintained until early August; in newborn male voles, only some of the males born in May had a similar testosterone level in the breeding season, while those that were born after June did not develop their gonads in their birth year. Those results indicate that overwintered males could breed throughout the whole breeding season; some of the early born males possibly took part in breeding, while those born after June could not breed until the next breeding season. Moreover, higher cortisol levels were found in overwintered male voles, especially in breeding season, which suggested that overwintered males have higher reproductive effort which caused a high stress level and mortality in breeding season. These results suggest that male voles born in different seasons have different patterns of gonadal development and reproductive strategies, which are related to their life-span expectation and imply the balance between existence and reproduction.