Aiming at the features of the kinematic coupling between the branches of parallel mechanism and the position problem of the branches bases in the world coordinate system, type synthesis of fully decoupled less DOF translational parallel mechanism was proposed. The basic concept of C set was explained and its algorithm was analyzed. The concrete steps of the type synthesis of fully decoupled less DOF translational parallel robot were presented by analyzing the composition characteristics of the mechanism based on the theory of C set. According to this theory, the three translational (3T) decoupling parallel mechanism synthesis was achieved and a large number of parallel mechanism groups with 3T motion characteristics were obtained. Then, taking the parallel mechanism of the traditional motion pair and the symmetrical structure as the main research type, the Jacobian matrix of the mechanism (3-PRRR) was analyzed. The validation of the method is demonstrated that the decoupling performance of the 3-PRRR is verified by ADAMS and the simulation result shows that the mechanism is completely decoupled.