The inservice oil of high speed train gear box will fail early under overvoltage condition,which will affect the train’s normal operation and maintenance.In this study,the bench experiment was used to simulate the failure of high speed train gear oil under overvoltage condition.And the corresponding relationship of overvoltage and breakdown time with chrominance,viscosity,acid number,product,and electrode damage of the oil were explored.The experimental results show that the critical breakdown voltage is about 1 500 V under the experimental conditions.When the experimental load voltage reaches 2 500 V and breakdown number reaches to 58 times,the product in the oil was significantly increased.A small amount of pitting corrosion occurred on the surface of the experimental electrode after the experiment.