In order to improve the accuracy and reliability of the transformer fault diagnosis, a fault diagnosis method of transformer based on MPC(modification of the PC, for short: MPC) algorithm optimized by Bayesian network was proposed, and the fault diagnosis technology of transformer was studied. Firstly, according to the analysis of dissolved gas in oil, the 9-D fault features of oil-immersed transformer were extracted by the non-coding ratio method, and the data samples were normalized. Secondly, a fault diagnosis model based on Bayesian network was established with normalized training samples as input, and the Bayesian network model was optimized with the MPC algorithm. Finally, the fault diagnosis model was tested with test samples. In order to prove the superiority of the proposed method, the proposed method was compared with the existing fault diagnosis methods. The result shows that the proposed method has higher diagnostic accuracy and better diagnostic effect.