With the increasing demand for green and clean energy in China and the rapid development of nuclear power industry, sedimentary rocks and softer rocks are used as the foundations for nuclear power plant sites. A nuclear power station with sedimentary rock foundation is analyzed as an example. Based on the data of the foundation rocks, including spatial distribution characteristics of lithology, the weathering degree, integrity of rock mass, physical and mechanical parameters of rock mass, etc., reasonable division of the engineering geological unit of the foundation rock mass is conducted through statistical induction. The problem in geological units division of sedimentary rock foundation for nuclear power plant is effectively solved. This study reveals the real engineering characteristics of foundation rock mass, and provides scientific basis for foundation design of nuclear power plant.