Abstract: Starting from the catenary power supply design and EMU (electric multiple unit) traction system structure, the causes of vehicle roof voltage transformer burnout failure of high-speed railway EMU in China are analyzed, and the 25 kV voltage transformer currently in service that fails to meet the fortification requirements for the high-speed railway traction catenary power pollution in China is detected as the main cause. Targeting this cause, it is proposed to increase the setting value of DC bias voltage from 1 500 V to 2 500 V, thus improving the voltage transformer ability to resist DC bias. The specific technical means is to cut part of the air gap on the core magnetic circuit of the voltage transformer, so that the primary winding of the voltage transformer can be converted into core reactor operation in case of DC bias, and the primary current can be limited to safe operation level. According to the test verification results, the improved voltage transformer can meet the protection requirements of 2 500 V DC bias voltage.