In order to evaluate the impacts of hydropower development on the ecological environment, the impacts of the cascade development of hydropower projects in the main stream of the Tongtian River on the terrestrial ecological environment were analyzed and evaluated using the “3S” technology combined with the methods of landscape ecology and the ecological security index, as well as data from digital images and field surveys. The results show that, subsequent to cascade hydropower development, the water area increased by 300.72 km², the grassland area decreased by 296.80 km², and there was no significant change in the main vegetation types and land use patterns. Grassland was the dominant landscape type in the basin, followed by bare land and woodland. Landscape fragmentation decreased after hydropower development, while landscape dominance and diversity increased, which was conducive to the stability of regional ecosystem. Cascade hydropower development did not significantly affect the productivity of vegetation in the basin and the richness of species on both sides of the river. The productivity of vegetation decreased by 1.83×10⁵ t/a, and the productivity after the development only accounted for 0.78% of that before the development. The ecological security index of the overall basin was 0.819 after the development, 0.001 higher than that before it, indicating a high ecological security index. In summary, since there were no major changes in landscape patterns or in ecosystem features and functions, cascade hydropower development does not pose a threat to the ecological environment and safety in the Tongtian River Basin.