Based on the data from five surveys of fish resources during 2018-2020 and literature, the fish composition and species diversity in Xishuangbanna reach of Lancang River are analyzed. The results show that there are 11 orders, 28 families, 79 genera and 119 species of fish being collected, while a total of 179 valid fish species are recorded, belonging to 12 orders, 34 families and 106 genera. With the deepening of the investigation, the number of indigenous fish species is increasing, but the proportion is decreasing; while the number of exotic fish species is increasing, and the proportion is increasing. In these surveys, 92 species of indigenous fish are found, accounting for 77.31% of the total number of fish species, and the proportion decreases further. 27 species of exotic fish are collected, accounting for 22.69%, the proportion increases accordingly. The dominant species of fish are Poropuntius huangchuchieni and Scaphiodonichthys acanthopterus, and common species are Mystacoleucus marginatus and Channa gachua. Five fish species are listed on the List of Key Protected Wild Animals in China (2021) and eight species as threatened species in the Redlist of China's Biodiversity:Vertebrate (2015). Fish species diversity in Xishuangbanna reach of Lancang River is extremely high among river of China. The fish species diversity of the main stem of Lancang River is lower than that of tributaries, while that of the left bank tributary is higher than the right bank, and that of the Luosuo River is higher than that of other tributaries. Although the fish species diversity maintains a high level in Xishuangbanna reach of Lancang River, the proportion of indigenous fish is declining obviously and it is still facing serious threats. The threats are analyzed and some suggestions on conservation are proposed.