Abstract: In order to achieve high efficient operation between the APU and the whole vehicle, control system of the auxiliary power unit has been developed. The structure, function, and working principle of the auxiliary power unit's control system have been introduced in this paper. The control system of auxiliary power unit adopts the INTEL 16 bit microcontroller to its control kernel and estimates the battery pack's SOC by inspecting the battery pack's voltage and the current. When the S()C is below the set value, the engine-generator system starts running at a steady revolution and chargs the battery pack. The auxiliary power unit' s control system can judge the auxiliary hybrid electric vehicle's working model, and drive the LCD mounted in the vehicle to display the vehicle's working model and battery pack's voltage and current value.