To obtain fully isotropic and decoupled structures of parallel robot, a very simple but very effective structural design is proposed based on G_F set. The basic concept of G_F set, algorithms and the type synthesis principle, requirements for rotation based on G_F set are firstly introduced. Secondly the selection criterion of the input pair and type synthesis principle of decoupled branches are given, therefore it ensures that the various branches' input are isotropic and decoupled. According to the type synthesis theory, structural synthesis of 2T2R four degrees of freedom decoupled parallel mechanism is finished. Simultaneously, a lot of new mechanisms are attained. Finally, the screw theory is applied to analyze kinematic characteristic of a parallel mechanism synthesized above. The expression of the Jacobian matrix is deduced which validated the decoupling feature of the mechanism. In addition it demonstrates the effectiveness of the novel method of structural synthesis for parallel mechanisms.