It is required that the aero-engine starter/generator system can start reliably when the load torque is unknown and the mutation is obvious in case of heat engine and cold engine. The starting time is also required to be as short as possible too. The traditional open-loop I/f control method of permanent magnet synchronous motor has some problems， such as long speed adjustment time， low current utilization rate， and easy out-of-step phenomenon when the given current does not match the load torque， so it is not suitable for the aero-engine starter/generator system. In this paper， an improved closed-loop I/f control strategy is proposed based on instantaneous power disturbance detection. By detecting instantaneous active power， the frequency of the given current vector is compensated， the damping torque component of the system is increased， and the speed convergence is accelerated. By detecting instantaneous reactive power， the amplitude of current vector is adjusted to keep the motor working in the state of maximum torque/current ratio and adapt to the sudden change of load torque. At the same time， a linearized model is established based on small signals. The stability and robustness of the traditional and the improved I/f control methods are compared， and the appropriate damping compensation coefficient is selected. Finally， it is verified through simulation and experiment that the improved closed-loop I/f starting control method can reduce the speed fluctuation by ±60 r/min in the acceleration stage， reduce the speed convergence time by 0. 3 s， and improve the current utilization rate by about 20%. The improved method can also adapt to the unknown and sudden load torque change in the aero-engine starter/generator system. When the given current is adjusted in time， the out-of-step phenomenon will no longer occur and reliable starting can be realized. The results show that the improved method can be effectively applied in the aero-engine starter/generator system.