The invention discloses an open-circuit fault diagnosis method of a cascaded STATCOM system IGBT. A capacitance and voltage signal with a semi fundamental wave period serves as an original signal to be processed, features of fault signals can be maintained, and diagnosis instantaneity can also be guaranteed. By means of a multi-core least square support vector machine improved through a feature dimension reduction, diagnosis accuracy is improved, and diagnosis time is shortened. The method further reduces misdiagnosis caused by load changes, and inherits the small-sample learning advantage of a traditional SVM classification method.