Abstract: In order to apply three-element mixed insulation oil to transformers with higher voltage levels, a simulation model of three-element mixed insulation oil streamer discharge was established on the basis of traditional fluid dynamics drift diffusion model, considering the influences of oil chemical composition on oil ionization energy and the variation law of oil ionization energy with electric field intensity. The characteristics of streamer discharge development of three-element mixed insulation oil under different application conditions were analyzed by simulation, and the effects of mixing balance, size and polarity were discussed. The results show that the three-element mixed insulation oil streamer discharge model established in this paper can better reflect the development morphology of oil streamer discharge. The larger the positive polarity voltage applied, the earlier the time of the surge in the development length of the three-element mixed insulation oil stream discharge. With the decrease of the tip curvature radius, the development speed of the streamer discharge is faster, but the diameter of the main channel streamer discharge is smaller. With the decrease of the oil gap distance between the pin-plate electrodes, the development of the three-element mixed insulation oil streamer discharge becomes more severe under the same voltage. Compared with the development characteristics of mineral oil, soya-based natural ester and palmit-based natural ester, the mixing balance effect dominated by oil composition is presented in the development process of three-element mixed insulation oil.