To research the distribution of temperature filed and the thermal stress of ULF light rail trains, the three-dimensional finite element model was constructed, and according to apply proper boundary condition to the brake disc , the heat flux generation and the heat dissipation of brake disc were simulated and computed by ABAQUS. The result showed that in the process of brake, the temperature of brake disc rose severely at first and then decreased with time, and increased in the radial direction without steady-state heat transfer. The distribution of thermal stress was related to the temperature field.However,the thermal stress of the brake disk increased after the inner circumference decreased first. The maximal thermal stress appeared in the edge of the brake disc. Therefore, the weak part is its edge for brake disc.