The invention discloses a method for detecting the breakage of a PT neutral line. The method comprises a step of calculating a positive sequence fundamental voltage and various higher-harmonic voltages of each phase according to voltage sampling values of A, B and C phases of a secondary side of a PT, a step of calculating a zero-sequence third-harmonic voltage according to the zero-sequence voltage sampling value of the secondary side of the PT, a step of calculating and obtaining a voltage threshold 1 according to the positive sequence fundamental voltage, a step of calculating and obtaininga voltage threshold 2 according to the various higher-harmonic voltages of each phase, and a step of reporting that the PT neutral line is broken after a delay when the zero-sequence third-harmonic voltage is larger than the voltage threshold 1 and the zero-sequence third-harmonic voltage is larger than the voltage threshold 2. By using the method, the breakage of the PT neutral line can be judged, when various higher-harmonic voltages are generated due to operations of no-load main transformer and the like in a system, besides third-harmonic content is large, other-harmonic content is also large, and the misjudgment of the breakage of the PT neutral line can be prevented.