The Sino-Mongolian beaver Castor fiber birulai lives in the Ulungur watershed in China and Mongolia, an area little known to the people outside this region. We recorded the number of families, adults, subadults and juveniles at each of three beaver lodges in the Bulgan Beaver Nature Reserve in 2003, 2006 and 2007. Along the whole Ulungur watershed in China in each of the 3 years we conducted surveys for the beaver and estimated the total population based on the area of food caches. We recorded 135, 167 and 145 Sino-Mongolian beaver colonies and estimated a population 472–599, 543–700 and 508–645 beavers in 2003, 2006 and 2007, respectively. From 1989 to 2007 the number of human households in the Bulgan Nature Reserve increased by 112% and the population by 71%. Consequently human activities in the river valley, including collection of wood for fuel, increased. We also surveyed the site of a relocation of C. f. birulai from the Bulgan River to the Ertix River in 1992 but found no sign of living beavers. The population of the Sino-Mongolian beaver in China is small and restricted geographically. Threats, such as habitat modification and deterioration and competition with people for wood, continue. We recommend that the Bulgan Beaver Nature Reserve be expanded by a transfrontier agreement with the appropriate authorities in Mongolia, and that a plan for sustainable wetland management and restoration is required.