The technology of controlled, self-sustained chain fission reaction is one of the most important applications of nuclear power, among which the water-cooled, thermal spectrum nuclear reactor is the dominant technique. The digital I&C system of a water-cooled of nuclear power plants is the nerve center of nuclear power plants and improving its reliability is of great significance to nuclear safety. In order to solve the reliability problems of the dynamic behaviors such as standby automatic switching and startup failure realized by the nuclear power plant digital I&C system, this paper studies the dynamic fault tree analysis method of the chemical and volume system. According to the control system structure involved in the charging control and the control logic of the charging pump cold and hot standby, a dynamic fault tree model of the upper charging function failure is established. The minimum cut set method and Markov model are used for reliability quantitative analysis. Reliability analysis results show that redundancy and backup improve the reliability of the system. The result of importance analysis show that for system optimization design, priority should be given to electrical switch cabinets and junction boxes, and electrical switch cabinets and process control cabinets should be given priority to inspection and maintenance.