Power transformer is one of the most important electric equipments in power network, its running state has a direct effect on its safe operation of power network. Fault’s occurrence of power transformer probably result in vast hazards or accidents. According to statistics, its over-temperature operation of winding often result in different fault patterns, such as aging, breakdown and its burnt, etc, which has a large proportion in its accidents of power transformer. In this paper, based on the thermal analysis theory, a 3D thermal field model of transformer’s winding and core is respectively established in ANSYS software environment. And its 3D temperature field distribution of power transformer, including winding and core is simulated. Numerical simulated results provide a reliable and convenient reference for its thermal performance analysis of power transformer.is simulated. Numerical simulated results provide a reliable and convenient reference for its thermal performance analysis of power transformer.