Stiffness is one of the most important performance factors that should be considered for parallel mechanism. A semianalytical approach for the stiffness analysis of a 3-R[TXX-]RS parallel mechanism was presented with consideration of gravity. The force analysis was carried out by taking both gravity of all moving components and the externally applied wrench imposed upon the platform into account. The deflection analysis that considered the distributed gravity of all limbs and joint/link compliances was investigated. On the basis of the formulation of component stiffness matrices in the joint space, the stiffness model of the parallel mechanism was achieved. Based on a numerical example, the stiffness distributions and deflection distributions induced by gravity throughout the entire task workspace were evaluated, and the contributions of the component stiffness to the global stiffness and the contributions of the components gravity to platform deflection were investigated. Meanwhile, the effectiveness of the proposed modeling approach was verified by the FEA software at a typical configuration. Numerical example showed that in order to make the parallel mechanism with lightweight yet rigid design, the rigidities of different components, as well as the actuated and constraint rigidities of each component should be matched each other. For the considered 3-R[TXX-]RS parallel mechanism, the rigidity of the spherical joint should be improved while the rigidity of the revolute joint should be reduced, the actuated rigidity of the limb should be enhanced while the constraint rigidities of that should be reduced.