Abstract: The spacecraft is in a complicated mechanical environment, and its external load can not be measured directly. So a mechanical load identification method is proposed for spacecraft during the flight phase. Based on the Duhamel's integration, with the second order polynomial as the base function, the dynamic load identification mode for a single-degree-of-freedom vibration system is built. The simulation results show that this method has a high identification precision and the problem of error accumulation is solved. The method can provide theoretical bases and technological supports for the test validation and engineering applications.