A method of dry resistance load test of diesel locomotive is introduced. The electric power generated by a dynamotor, which is driven by the diesel, is converted to DC and consumed on the dry resistances. As the heat dissipation of the resistances is about several thousand kilowatts, DC motor fans are used for air cooling the resistances. In order to save energy, the energy for driven the motor fans is come from the electric power of the DC power. A constant current speed regulating system for the motor fans is presented, which bases on a kind of high-power switch component (IGBT). The experimental results are discussed.