By using Kane Equation, the dynamic characteristic of the multi-dimensional loading device was analyzed, and dynamic mathematical model was established. Aimed at the different parameters of motion about the loading device, the effect of the gravity, inertial force and the Coriolis force in the dynamic model to the generalized force of system output was analyzed with simulation, under conditions of different acceleration and velocity. The results show that when the velocity or acceleration of the moving platform is within the working limits, the effect of the inertia force branch and the Coriolis force to the system is smaller than 2%, can be omitted, while the gravity of that is greater than 2%，can not. A simplified system dynamic model is gotten according to the analyzed results, which provides the theoretical basis for the control system of parallel device.