The impact of cascade hydropower development on the ecological environment have been studied from the aspects of hydrology, water environment, biology and soil erosion.The paper predicted and evaluated the eight environmental affecting factors of three sections in second Songhua river basin by use of GM (1,1) model. It turned out that cascade development of river hydropower is benefit to the increase of fish resources quantity and the decrease of sediment discharge, decreases annual average runoff to a certain extent and has negative influence on the protection of fish species. Therefore, the cascade hydropower development need careful planning and ration scheduling,the site selection,dam height, and impoundment of each level have great impact on the ecological security of the whole river basin.