Abundant experimental data have been obtained on the breakdown characteristics of transformer oil paper insulation under extremely low temperature at home and abroad. However, little attention has been paid to the research on temperature field and flow field of transformer cold start under extremely low temperature. In order to explore the distribution of temperature field and flow field of oil immersed transformer during cold start, SFZ11-10000/35 type 35 kV oil immersed air-cooled transformer produced by Yunnan Tongbian Electric Appliance Co., Ltd. was established as a two-dimensional axisymmetric heat conduction model by using COMSOL. The temperature field and flow field in the cold starting process of transformer under extremely low temperature were simulated. The correctness of the numerical calculation was verified by designing experiments. The simulation results show that: during the cold start process, a number of small-scale oil flow circulation areas are formed above the core and winding. With the cold start, the small-scale oil flow circulation is continuously expanded and is fused into a large-scale oil flow circulation. After 420 minutes, the temperature field and flow field gradually tend to be stable. The maximum temperature is 64 ℃ and the maximum speed is 91 mm/s. The uneven temperature distribution and unstable oil flow in the transformer are caused by the large local temperature field and flow field gradient in the uneven transformer.