An approach for the lightweight design of a 3-R-RS (R and S represented revolute and spherical joints, respectively, and R- denoted the revolute joint is active) parallel mechanism was presented. The stiffness model of the 3-R-RS parallel mechanism was expressed as the stiffness model of two sub-assemblies so as to obtain conclusion that the 3-R-RS parallel mechanism can be regarded as connected by the two sub-assemblies ‘in series’. The stiffness matching coefficients between the two sub-assemblies was obtained by maximizing the lowerorder natural frequencies, and then the rigidity constraints of the 3-R-RS parallel mechanism can be allocated to the two sub-assemblies. On this basis, the lightweight design of the 3-R-RS parallel mechanism can be developed by respectively minimizing the weights of the two subassemblies subject to technological processes, geometric interference constraints and the specified rigidity constraints attributed to them. It should be pointed out that an iteration was adopted to obtain the stiffness matching coefficients, and the response surface fitting was used to fit relationships between the structural parameters of the limb body and its stiffness and mass due to the complexity of the shape of the limb body. The proposed approach simultaneously enabled the parallel mechanism to achieve both high static rigidities and high dynamic behaviors. Numerical example showed that the differences in the stiffness matching coefficient led to the differences in the distribution of the nature frequencies, which instructed that the stiffness matching coefficient had significant influence on the dynamic characteristics.