Abstract: Aiming at the effect of static stiffness on the performance of PMT (parallel machine tool), the 3-TPT parallel machine tool developed by NEU is taken as the target to investigate. Analyzing theoretically the static stiffness model of the machine tool, an expression is given relevantly. Then, the stiffness of both the main frame and the whole machine tool under different forcing conditions are analyzed simulatively using the software ANSYS for large-size FEA, thus showing the low-stiffness positions and the effect of parallel mechanism on the whole machine tool. The simulating result indicated that the stiffness of a machine tool in vertical direction far exceeds that in horizontal direction and the stiffness of the whole machine far exceeds that of the main frame. It implies that the parallel mechanism enables the stiffness of machine tool to be doubled, and that the stiffness increases with the height of the position where the parallel mechanism is laid on.