The invention discloses a fault diagnosis method of a photovoltaic diode clamping type three-level inverter. The fault diagnosis method of the photovoltaic inverter combining with a wavelet multi-scale decomposition method and a support vector machine classification algorithm is provided by aiming at a three-level inverter fault diagnosis problem of a photovoltaic micro-grid, taking a system inversion state as an example, and analyzing fault types of various power tubes. The fault diagnosis method adopts the bridge arm voltage signal of the diode clamping type three-level inverter as a research object, and various frequency band energies are extracted as fault characteristic samples by using the wavelet analysis multi-scale decomposition method, and in addition, a multiple valued support vector machine fault classification model is established, and finally, the open circuit fault diagnosis of the three-level inverter power device is completed. The fault diagnosis method is advantageous in that the various fault states of the diode clamping type three-level inverter are obviously discriminated, and the extraction is convenient, and in addition, problems of a conventional data sample diagnosis extraction method such as large data size and tedious process are overcome.