Inlet door position control subsystem of auxiliary power unit system is used for controlling inlet door opening and closing of auxiliary power unit on the ground and in the air. The control subsystem usually consists of the controller, the electrical actuator and the linkage mechanism. The inlet door position control subsystem of auxiliary power unit system is a part of the auxiliary power unit control system. The inlet door design, inlet door aerodynamic load analysis and inlet duct design are synchronal processing and interactional. This paper presents a inlet door position control design of auxiliary power unit system. This inlet door position control subsystem uses a separate controller for inlet door control, which make the APU FADEC hardware and software design complexity low and simplify the interface design. A new inlet door actuation mechanism is applied, whose installation and removal is easy and accessible. The inlet door actuation mechanism has the function of output torque amplification and the output torque is adjustable. This inlet door position control subsystem is applied on an aircraft model and demonstrated. The inlet door position control subsystem is a good reference to other aircraft model development.