Open phase fault monitoring in auxiliary transformer circuits is one of the most concerned issues in nuclear power plants. Open phase fault may cause abnormal conditions of plant power system in a nuclear power plant, such as unbalanced bus voltage, tripping of operating equipment, standby equipment failing to start, etc. If the high voltage side of the auxiliary transformer circuit fails to detect the open phase fault in time under the normal operation mode of the nuclear power plant, once the generator bus loses voltage, it is necessary to switch to the auxiliary power supply outside the plant, which may lead to the tripping of critical motors and even the loss of system safety function, so that the nuclear power plant cannot be shut down safely. Aiming at the commonly used auxiliary transformer schemes in China, this paper puts forward two schemes of monitoring open phase fault. Through the analysis of open phase monitoring scheme, and using PSCAD/EMTDC software modeling and simulation, this paper analyzes the characteristics of two schemes. It is concluded that how to select an open phase monitoring scheme for different auxiliary transformer schemes in nuclear power plants can more accurately and effectively identify the open phase fault.