The invention discloses a phase change failure prevention method based on rectification side trigger angle emergency control. Occurrence of phase change failure has a unique correspondence relationship with the rectification side trigger angle and the inversion side trigger angle when the DC system primary parameter and the control system and AC system voltage are certain under the premise of reasonable assumption according to mechanism analysis of phase change, and the rectification side trigger angle and the inversion side trigger angle have no coupling relationship so that the possibility of phase change failure can be rapidly reduced by changing the rectification side trigger angle according to certain logic after the fault occurs in the AC system. The variable fault detection threshold and the variable inertial time constant are adopted, and the rectification side trigger angle is directly controlled without the PI control link so as to have the advantage of rapid response. According to the method, the possibility of phase change failure can be reduced and safe and stable operation of the AC-DC compound system can be maintained.