The invention provides a phase modifier fault diagnosis method. The phase modifier fault diagnosis method aims at the problem that most of the current equipment fault diagnosis depends on detailed analysis and processing of field-acquired signals by front-line professionals, and the real-time nature of fault detection and processing is limited in the mode; and considering that the phase modifier plays an important role in improving the direct current transmission power and the voltage stability of the sending and receiving grids, and in order to ensure the safe and reliable operation of the phase modifier, the phase modifier fault diagnosis method comprehensively uses forward and reverse hybrid reasoning, expert field diagnosis rules (a knowledge base) and a gray correlation analysis method to perform correlation and credibility calculations on input operating data and vibration data; therefore, the real-time property and accuracy of shaft system vibration fault diagnosis of the phasemodifier are improved, which has great significance for reducing the probability of equipment failure.