In this paper, taking a 31500kVA natural wind cooling transformer as the simulation object, and applying control volume method simulation analysis for 3D temperature field of the transformer are obtained. And maximum temperature and position with the different load and the different environmental temperature are gained, meanwhile the simulation results were compared and analyzed with calculated values and experimental values, it is concluded that the simulation results are closer to actual values, and analyze that load, environmental temperature and the radiator effect on the winding hot-spot temperature. This results have certain guiding significance for temperature measurement system of transformer.The simulation result obtain accurately and intuitively vertical temperature distribution in transformer winding, it shows three windings temperature distribution and obvious contrast. And winding hottest-spot are in the top position in the whole winding, and roughly around 80% winding height. Meanwhile by adding the radiator simulate different cooling conditions, it is concluded that temperature for different locations in the same winding (near interphase or the tank) is different, the interphase is a little higher, and for different cooling conditions, hot-spot position changes slightly.