Abstract: Firstly we made some simplifications and hypotheses to cascade hydropower stations and cascade hydropower station system, then analyzed the main components that have influence on seismic hazard of cascade hydropower station system. By taking all the cascade hydropower stations as one unified system and account of the additional seismic hazard from the nearby upstream dam, we propose a new method to assess the seismic hazard of cascade hydropower station system. Taking the change of seismic background after the Wenchuan MS8.0 earthquake into account, we took the cascade hydropower station system in the trunk stream of Dadu river as a case study and evaluated its system seismic hazard nowadays. Based on the characteristics of cascade hydropower station system and considering the technological and economical feasibilities in its planning, designing and constructing, we think that it is the most effective path to improve the system seismic safety by raising the design seismic parameters of the several weakest hydropower stations.