The nuclear power plant control rod drive mechanism power supply system (RAM) is the only power supply system of that mechanism. In order to analyze abnormal working conditions of the system and solve the problem of loss of field protection mismatch, first, a simulation model of a RAM system is established based on the PSCAD simulation platform. To verify the correctness of the model, the mechanism of abnormal working conditions is analyzed, and the correctness of the analysis is verified by simulation. Secondly, the mismatch between the loss of field protection and overcurrent protection in a nuclear power plant is simulated and replicated, and the causes of the mismatch are analyzed. Finally, the logic optimization and coordination optimization of the loss of field protection are carried out. A III section loss of field protection configuration scheme is developed. The applicability of the scheme is verified from two dimensions of different degrees of excitation failure occurring at different load levels. The analysis of the developed RAM system abnormal operating conditions and the proposed loss-field protection optimization are of guiding significance to improve the safety of nuclear power plants. This work is supported by the Fundamental Research Funds for the Central Universities (No. 2020JBM070).