Abstract: In order to address the problems of large calculation and low accuracy of Buck circuit fault diagnosis methods, a buck circuit fault diagnosis method based on digital twin is proposed. Firstly, the digital twin model of Buck circuit is established by Matlab/Simulink software platform, and the initial parameters of the digital twin model are set according to the nominal values of Buck circuit components.Secondly, the acquired Buck circuit output voltage signal and operation state are mapped into the digital twin model, and the objective functionsare established according to the digital twin model and the output voltage of Buck circuit. The Levenberg-Marquart algorithm is used toiterate and optimize the objective functions to achieve the digital twin modelupdate so as to realize the parameter estimation of Buck circuit components. Finally, the estimated parameters obtained from the digital twin model are compared with the nominal values of Buck circuit components. When the difference between the two exceeds 20% of the nominal values, it is indicated that the component is invalid and the Buck circuit fault diagnosis is obtained. The experimental results show that the proposed method has high estimation accuracy and diagnostic reliability for Buck circuit component parameters.