As an indispensible part of electronic equipment, the reliability of the whole system is affected by the degradation performance of power MOSFET tube. Based on geometry, material properties and boundary conditions, the repeated testing can be reduced, and the period of failure analysis can be shortened. This article is based on the finite element model of power MOSFET TO-263, electric thermal — mechanical coupling analysis is introduced. The steady and transient thermal analysis is given, and then the inelastic strain range of thermal stress is calculated Finally on the basis of these, simulation can be realized, thermal fatigue life of power MOSFET tube is predicted through the Coffin — Manson's law.