Abstract: Ball screw feed system is an important part of CNC machine tool, and its static/dynamic stiffness has an important effect on the machine tool performance and stability. In the working process, the static/dynamic stiffness of the ball screw feed system changes with the stroke. In order to study the static/dynamic stiffness characteristics of the ball screw feed system with different support modes, the lumped mass method was used to establish a dynamic model, and the static/dynamic stiffness variation of the system within the stroke was studied. The influence of the mass and placement of the workpiece on the static/dynamic stiffness of the system was also concerned. The re`sults show that in the negative stroke, the static/dynamic stiffness of the system with fixed supports at both ends decreases first and then increases with the increase of stroke; the static/dynamic stiffness of the system with one end fixed and one end floating support decreased continuously. Compared with the two, the dynamic stiffness of the system with two ends fixed support was more sensitive to the mass and placement of the workpiece, that is, the natural frequency changed relatively large. The results can be used to guide the optimal design of the ball screw feed system and the selection of the mass and placement of the workpiece during the working process.