Study was carried out about position relationship among input and output structural parts by aimed at the degree of freedom (DOF) and 4RRR parallel mechanism. Analysis of the DOF property of the mechanism was carried out by using the theory of the screw theory, and the position of driving pair was determined.The inverse and forward dynamics equation of the mechanism was built, and the orientation of the fixed motion platform was calculated numericallyso as to validate the correctness of equation built. The relationship between changing of rod length and work space size with motion decoupling of the mechanism was studied. Finally, the output turning angle of driving pair is simulated numerically under the fixed motion platform. 