Two dimentional mathematical model of tunnel temperature field in condition of diffusion during seam smoldering steady according to heat transfer theory and comservation of mass, momentum and energy. Temperature character of tunnel in condition of diffusion during seam smoldering steady is simulated by COMSOL Multiphysics according to this model. Results show that temperature field in tunnel can be divided into three fields such as isothermal region,one dimention temperature rising region and two dimentional temperature rising region,and temperature field equation of appropriate region is obtained. So that theary direction is provided for prevention and contral of seam smolder.