There exist hotspots, fatigue cracks, and stress fracture on friction surfaces of steel or iron brake disc during service, and these thermal damages greatly contribute to the fatigue crack failure of the friction surfaces. So, it is very significant for researching on the forming mechanism of thermal damages above to prevent the fatigue failure of the friction surfaces from occurring. The method used in the paper is to combine FEM analysis with theoretical analysis. Forming mechanism, influencing factors and preventive measures of the thermal damages above was studied by use of the method. The temperature condition and microstructure evolution of hotspots, the behavior of thermal crack initiation and propagation and the distribution of thermal stress contributed to five constraints all were focused on.