Abstract: The digital systems applied to nuclear power plants need to address the problems created by common-cause failure (CCF) of the software. The diversity actuation system (DAS) can resist and relieve accident consequence for nuclear power plant, when postulated CCF of reactor protection system and design-basis event take place at the same time. According to the defense-in-depth and diversity principles of nuclear power plant, the requirement analysis of DAS for pressurized water reactor was researched. The design characteristic property, design process, design basis, design methods and system structure of DAS were systematically expounded.