In this paper, through generally using basic priniciple of heat transfer and gear, established the transient conduction differential equation and its definite conditions. On the basis of tribology and engagement theory, established the periodical boundary condition. Using ANSYS Programm, we analyzed temperature distribution of a spur gear operating under transient friction heat, and its periodical change. Simultaneously, we get the flash temperature distribution on the engaged surface of spur gear and its change curves. The basis for precisely studying its thermal deformation and profile modification is also established.