2UPS-2RPS is a 4-DOF parallel mechanism with two different structure limbs. It has the advantages of simple mechanical structure, low manufacturing cost, and simple control algorithm. The number of its DOF is less than six, the limbs not only transmitting driving force but also constraint force at the same time. So both the required driving and constraint force analysis are necessary in the dynamic control and optimization design of the mechanism. Inverse dynamics equations of the mechanism were modeled by using the Newton-Euler formulation, through solving the equations, the required driving and constraint forces can be obtained when the motion of moving platform and working load were given. A computational example was also provided.