Abstract: This paper describes the real time dynamic simulation research on the digital speed control system of an aeroengine under small perturbation conditions. The control calculation approach has been programmed for a microcomputer. The proportional derivative control is applied to the large error region and the proportional control to the small error region. Furthermore, in the small error region, the large proportional constant control is adopted if the error increment tends to increase, and the small proportional costant control is employed when it tends to decrease. For the sake of normal operation in the digital speed control system, the influence of the high frequency disturbance, the overspeed and spillover in the microcomputer have been eliminated. Finally, the flow chart of the program and the real time simulation tests are demonstrated. The test results and some useful conclusions are also presented.