The requirements, principles and steps of building reliability index system of the nuclear power system are proposed in this paper. First, due to the particularity of the nuclear power system, its reliability index system must meet the requirements of any different person, different levels, different factors and general engineering. Then, the principles of integrity, coordination, necessity, consistency, operability, industry characteristics and standardability should be met when establishing the reliability index system of the nuclear power system. Finally, the construction of the reliability index system of nuclear power system should follow the correct steps. The Control Rod Drive Mechanism (CRDM) is taken as an example to construct the reliability index system in this paper, which lays a foundation for the allocation and prediction of reliability indicators.