The (RPa∥3R)2R+RPa mechanism is an asymmetric 3DOF translational parallel mechanism（PM）designed by the author’s team based on the POC equation of the topological structure design theory. This parallel manipulator has zero coupling degree and partial motion decoupling is also easy to be manufactured. The stiffness model and performance of the PM were studied. Firstly, the stiffness model of the limb was established by using the virtual spring method, and the static equation of the limb was given. Then the overall stiffness distribution of the PM in the working space was given, and the torsion and linear stiffness of the x, y and z axes were analyzed respectively. Furthermore, finite element analysis method was used to analyze the deformation of the PM, and the deformation results of the PM were compared with those obtained by the virtual spring method, the correctness of stiffness results was verified. Finally, the stiffness characteristics of (RPa∥3R)2R+RPa mechanism and Delta mechanism were compared with different sections, which indicated that the stiffness value of (RPa∥3R)2R+RPa PM was greater than that of the Delta PM. The research result laid a foundation for the structural design of the parallel manipulator, and also provided a systematic theoretical framework to evaluate the kinematic properties of such PMs as a basis for future dimensional optimization, prototype development and dynamics analysis.