The process of weaning in primates means that infants start acquiring nutrition independently, it also reflects the regulation of female reproduction and the adaptive mechanism of weaning behavior under specific environmental conditions. To understand the weaning process and their influencing factors in Black Snub-nosed Monkey (Rhinopithecus bieti) infants, we observed a group of infants aged 1 to 26 months living in Mt. Lasha, within Yunling Provicial Nature Reserve, Lanping, Yunnan, China, from May 2008 to April 2010. Using the 10-minutes scan sampling method, we analyzed the behavioral development of the infants monkeys as indicated by their activity time distribution and parental care (body contact and distance), and determined the weaning process pattern. During the first 12 months of age, infants were in physical contact with adult females for almost half of the daytime (49.59%), mostly being carried (47.07%). The feeding time of was 8.45%, and the grooming time for other individuals was 0.09%. Between 13 to 17 months of age, the average time of parental care (physical contact) by adult females decreased to 19.62% (carrying 4.74%), while feeding time more than doubled (22.06%), and grooming time for other individuals increased slightly (0.96%). Between 18 to 26 months, infants spent less time in direct contact with adult females (17.18%, carrying 0.82%), while feeding and grooming with other individuals kept increasing (38.48% and 1.39%, respectively). Based on behavioral development of infants and adult females’ care decreased, we determined that the weaning process of R. bieti infants happened between their 13th and 17th months of age. This period corresponded with the season where high-quality food was available, favorizing the adaptation and survival of the newly independent infants, as well as boosting the recovery of mothers to support the next generation. These findings suggest that the weaning of R. bieti infants is mainly regulated by adult females’ care and food availability.