Based on Udwadia-Kalaba theory，a method for calculating the dynamic load of suspension part was studied by taking the six-component force of wheel center as the input. Taking the five-rod rear suspension as the modeling object，the dynamic model of the suspension system was established，and the analytical expression of the load at each part was obtained with the decomposition of the total binding force. Compared with the traditional Lagrange modeling method，the method does not need the Lagrange multiplier and reduces the complexity of modeling. In order to verify the correctness of the method，the numerical results were compared with the simulation results of the traditional modeling software Adams/Car. The analysis result shows that the proposed method is accurate and efficient，and the calculated dynamic loads are in good agreement with the Adams/Car simulation results. Finally，the method was applied to the actual vehicle test，and the load prediction for the rear suspension of a SUV was carried out.