Abstract: The identification of the complicated mechanical loads in the ascent stage of the launch vehicle & spacecraft combination is essential for the study of the mechanical environment and the test condition design. In view of the inadequacy of the present ability of directly measuring the thrust pulsation and the transonic buffet, a method is put forward to inverting the mechanical load of the launch vehicle and spacecraft combination during the powered flight phase into other forms. Based on Duhamel’s integration and the mode superposition algorithm, and with the help of quadratic polynomials, an arithmetic model is established for identifying the dynamic load of the multi-DOF vibration system. The result is compared with that obtained by the the precise time-integration method. It is shown that the new method enjoys a desirable anti-jamming capability(robustness) and a high precision of identification. Meanwhile, the problem of error accumulation is solved. By increasing the sampling frequency or decreasing the time step, more accurate results can be obtained, to provide a technological support for the related engineering applications.