The inertia value of the motion simulator of the docking performance testing platform (‘performance testing platform’ for short), applied for docking mechanism ground docking performance tests used in Lunar Exploration Program, can affect the accuracy of ground test data. As the inertia of the motion simulators for the performance test-bed must be exactly the same as those of the spacecrafts which the test-bed simulated, there is the great practical significance to measure the inertia of the motion simulators in-situ. In this paper, the motion vibration equation and in-situ calibration method of rotational inertia for the motion simulators is firstly investigated based on the additional mass spring oscillator method. Secondly, the mechanism motion models of the motion simulators are built based on SimMechanics Tollbox Software before and after attaching additional counterweights, the vibration periods of the two models are simulated, and the simulation inertia is calculated based on the theoretical model and the simulated vibration periods, verifying the correctness of the theoretical algorithm. Finally, a calibration device based on voice coil motor and limit mechanism is established for in-situ calibration tests of the motion simulator and analyze the main sources of errors in the tests. Experimental results show that the calibration results of the inertia of the three axes have a deviation of no more than 5% compared to the theoretical values, and the measurement error caused by the device is less than 2.5%, which is less than 1/4 of the maximum permissible error of the motion simulators. The results indicate that the calibration device can solve the problem of inertia calibration for the motion simulators of the performance testing platform.