The authors of the paper have developed a program to describe the performance of heat generation for this kind of generator. The heat conduction differential equations, which accord with the actual situation, to describe the temperature distribution of the stator and the rotor were discretized by finite difference method. An alternative-direction-implicit(ADI) line iteration method was carried out in the program. On this base, the authors analysed the various factors which influence the temperature rise of the generator set and provided a way for solving the problem of the overheating of the small-sized direct current generator. The results indicated that the numerical calculation values are goodly agreed with the experimental data and that the program is realiable.