The ecological impact of a species distributed in a new area would be uncertain to local ecosystem, so it is important to monitor its population dynamics for developing appropriate management strategies. The Asian Openbill (Anastomus oscitans), a newly recorded species in China, have spread to Yunnan, Guizhou, Guangxi and Sichuan provinces in recent years, however, its population status and trend were still lack of understanding. We conducted population census on the Asian Openbill in Caohai National Nature Reserve in Guizhou province (26°47′﹣26°52′N, 104°10′﹣104°20′E) during January 2013 to December 2014 by counting their night roosting sites (Fig. 1). Caihai wetland is a typical plateau lake wetland surrounded by shallow water mashes in southwestern China with an altitude of 2 171.7 m above sea level. We carried out the counting surveys one time per week at sunrise, with 5 groups responsible for the 5 night roosting sites. Our results showed that: 1) The Asian Openbill stayed in Caohai Nature Reserve mainly in summer and its population trend exhibited significant seasonal fluctuations (P < 0.001). 2) The population size of Asian openbills increased rapidly (Fig. 2), with the average of 200.6 ind (SD = 94.6, n = 24, Max 427 ind) in summer of 2013 and 356.1 ind (SD = 180.3, n = 25, Max 602 ind) in summer of 2014. 3) The Asian openbills in Caohai roosted in trees at night, as the 5 night roosting sites, the Bojiwan area, located in the southeast of Caohai, was the main place accounted for 56.46% and 63.53% of the whole population in 2013 and 2014, respectively (Table1). We suggest to establish a monitoring network consisted by the forestry departments and nature reserves to monitor the whole population of the Asian Openbill in China, including population size, distribution etc.