Abstract: The analog control system of Daya Bay Nuclear Power Plant consists of discrete electrical components integrated by hard wiring, and it is planned to undergo digital transformation during the 30a overhaul. Due to the functional limitation of the analog platform, the single failure of equipment has a great impact on the control loop, and a large number of instrumentation and control devices are identified as critical components. Reducing the number of critical components is a key goal of the Daya Bay Nuclear Power Plant Digital Transformation Project. The Project takes pressurizer water level control loop—the important control loop of nuclear island as the research object and systematically studies and practically applies the degradation measures of critical components. By using the automatic voting function and fault diagnosis function of the reconstructed digital control system (DCS), the voting optimization scheme of measurement channel and the double redundancy design scheme of output channel are put forward. Using the principle of interface optimization, main control unit and auxiliary control unit of the pressurizer water level control loop are concentrated in a functional subgroup, and a large number of cross-subgroup interfaces are cancelled. With the application of the above measures, the number of critical components for instrument control of the pressurizer water level control loop has been reduced from 17 to 0, which has comprehensively improved the reliability of control functions, reduced the management cost of plant equipment and and provided an important reference scheme for achieving the key objectives of the Project.