Based on the traditional stiffness mapping matrix, a stiffness mapping model of modified Delta mechanism was established. The model considered both the influence of stiffness of active joints and external load and the influence of gravities factor and all parts’elastic deformation under the constraint force on the mechanism stiffness so that it was more general and exact than any other stiffness models. Evaluating the stiffness characteristic of parallel mechanism by stiffness matrix form was not intuitive. An idea was proposed to evaluate the stiffness performance by the elastic deformation of the mechanism. The static constrained forces of all joints were calculated by using the principle of virtual work. The elastic deformation under the constrained force and gravity was calculated by superposition principle. At last, the comprehensive elastic deformation of the mechanism was obtained by coordinate transformation method. The stiffness characteristic of the mechanism was further analyzed.