Parallel mechanism with its advantages of high stiffness, high bearing capacity and high precision fills the shortcomings of the series mechanism. For a 3RRPaR parallel mechanism with redundant structure, a dynamic model was established based on Lagrange multiplier method, and then the dynamic analysis of the redundant parallel mechanism was carried out. Firstly, the structural characteristics of 3RRPaR parallel mechanism were analyzed, and the redundant structure was determined. Then the inverse kinematics model of the parallel mechanism was established by closedloop vector method, and the dynamic model of the parallel mechanism was established by Lagrange multiplier method. Finally, the dynamic equations of the parallel mechanism under noload and loading were solved by Matlab, and the dynamic response curves were drawn. The driving moment of the driving rod was calculated according to the relationship between the Lagrange multiplier and the constraint moment. The comparison between the calculation results and the ADAMS virtual simulation results verified the correctness of the dynamic modeling method. The results showed that the driving torque of the driving rod was increased with the loading of the moving platform, but the numerical variation law was basically the same as that of noload. The research laid a foundation for the dynamic control and mechanism performance research of the parallel mechanism, and also provided a method and idea for the rigid body dynamic modeling of other redundant parallel mechanisms. 