Abstract: The abnormal speed fluctuation of the external shaft was found in a thermal power system test. We systematically analyze the working properties of engine，electric generator, vehicle shell and rudder actuator, which are closely connected with the external shaft of engine, and establish a mathematic model to support the analysis and estimation. The conclusion is drawn that the rotational speed fluctuation of the engine external shaft is caused by the change of the electrical power load on electric devices, and this change of the electrical power load also disturb the roll control circuit. The improper response to the disturbance causes frequent movement of the differential rudder actuator, which makes the rotational speed of engine external can not get complete recovery in the end.