The No.264 Important Bird Area (IBA) situates at Mengzi Basin, Yunnan. However, the data on avian diversity is deficient. From 2006 to 2014, bird surveys had been conducted around three lakes (the Changqiaohai, the Datunhai, and the Sanjiaohai Lake) in Mengzi Basin, southeast Yunnan, during late January and early February. During the study period, waterbird monitoring by direct census was conducted by the same team at 12 sites around the three lakes. The number of waterbird species, density, Anseriformes birds, Ciconiformes birds and coot were applied to Man-Kendall (MK) test to reveal the trends over the study period. The Shannon-Wiener, Pielou and Dominance indices of the waterbirds in each lake were calculated. 63 species of water birds belonging to 33 genus of 12 families had been recorded in totall. There were 1 species of 1st class national protected animals (Ciconia nigra), 2 species of 2nd class national protected animals (Platalea leucorodia and Plegadis falcinellus), 1 Critically Endangered species (Aythya baeri), and 4 Near-Threatened species. The Changqiaohai Lake accommodated the most numerous species and the largest number of individuals, but the Shannon-Wiener index value is low and the Dominance Index value is high. The Sanjiaohai Lake accommodated numerous species, and the Shannon-Wiener index value is high and the Dominance Index value is low. Few water birds occurred at the Datunhai Lake. The results of MK test revealed that no significant change occurred to the species number and density of individual waterbird in the three lakes, while the number of Anseriformes birds showed a slight downwards trend and the number of Ciconiformes birds showed a significant downwards trend. The number of coot showed no obvious trend. Considering the protected species and large flocks of ciconiformes birds occurring at the Sanjiaohai Lake, the range of No. 264 IBA should be extended.