At present, design standards of nuclear power plant in domestic and foreign only consider the impact of complete voltage loss on the operation of nuclear power plants, while ignoring the challenge of open phase fault to nuclear safety. The electrical parameters of auxiliary transformer are very weak when an open phase fault occurs under no-load operation, so how to detect and distinguish the open phase fault is the most concerned problem in nuclear power plant. This paper focuses on the open phase fault of auxiliary transformer during no-load condition. By combining the symmetrical component method and transformer principle, the electrical parameter characteristics of single-phase open phase on the high voltage side of auxiliary transformer under no-load condition are analyzed. The simulation model of the power system of HPR1000 nuclear power plant is established and analyzed by using ETAP software. Based on the results of theoretical analysis and simulation analysis, the countermeasures of single-phase open phase fault on the high-voltage side of the auxiliary transformer are put forward, which improves the power supply reliability of the auxiliary power supply outside the plant and is conducive to the safe and orderly operation of the nuclear power plant. In this paper, the research and analysis results of the electrical parameter characteristics of the open phase fault on the high voltage side of the auxiliary transformer under no-load state have guiding significance for the configuration of the open phase protection schemes for auxiliary transformers and the development of the open phase protection equipment.