Due to rich hydropower resources, Lancang River has been intensively dammed for hydropower production. In this study, we investigated variations of fish communities in the mainstream of Lancang River during cascade hydropower development and explored the underlying mechanisms by collecting historical data and conducting field surveys. The results demonstrated that, as cascade hydropower developed, fishing production gradually increased and exhibited a significantly positive relationship with installed hydropower capacity (r=0.936, P<0.01), which has increased by approximately eight times until now, reaching 53930 ton. However, fish diversity significantly decreased at the beginning of hydropower development due to the disappear of native fish, but then recovered to the initial level due to exotic fish invasion. The fish diversity index of D_G-F was 0.67, 0.47 and 0.67 before 1990, 2008-2013, and 2018-2019, respectively. During cascade hydropower development, the main ecological types of fish gradually changed from riffle to slow current, and from demersal eggs to adhesive eggs; the proportion of herbivorous and filter feeder also gradually increased. After hydropower dam constructions, the flow velocity decrease and nutrient settlement created favorable conditions for phytoplankton proliferations, which contributed to fish abundance increase. The change of ecological environment and stress from invasive fish caused the shift of fish ecological types. This study will benefit the evaluation of ecological impacts of hydropower exploitation and fish diversity protection in dammed rivers.