The invention discloses a synchronous phase modifier fault diagnosis method based on longitudinal analysis and lateral correction. A fixed value longitudinal numerical analysis method is used to determine whether the terminal of a single phase modifier has a single phase ground fault, and the fault is located. An average mean lateral screening method or a lateral bubbling screening comparison method is used to determine the excitation voltage disturbance of multiple synchronous phase modifiers and locate the disturbance. According to the invention, longitudinal and lateral data of the phase modifiers are comprehensively analyzed to determine the occurrence of the disturbance and the influence of the disturbance; the accuracy of fault diagnosis and positioning is improved; and the operationand maintenance efficiency of a maintenance department is effectively improved.