The thermal fatigue damage of the brake disc caused by the thermal load of the friction brake when the electric brake fails is an important factor for the safety of train operation. A three-dimensional finite element model of the friction brake of the axle-mounted brake disc of the subway train was developed. The thermal-mechanical coupling of the brake disc in service brake and one emergency brake was investigated, and the temperature field and stress field of the brake disc under the two braking modes were obtained. Results show that the temperature distributions of the brake disc surface have similarities under different working conditions. In the beginning of braking, the temperature on the disc surface rises rapidly and quickly reaches the peak points of 166.81℃ and 151.5℃. Then the disc surface temperature decreases slowly. The initial stress field is dominated by mechanical stress. With the increase of braking temperature, thermal stress becomes the main influencing factor. The stress field has the similar distribution with the temperature field, but the stress peak appears later than the temperature peak. The thermal stress will cause the accumulation of material damage in the braking process which leads to the fatigue cracking of the brake disc.