Abstract: As the core part of the machine tool spindle, the dynamic stiffness of the bearing is very important to the machining accuracy of the machine tool. In order to simulate the working conditions of the machine tool spindle bearing and measure the dynamic stiffness of the bearing during its operation, a bearing dynamic stiffness measuring instrument was designed, which can measure the axial or radial dynamic stiffness of the bearings in various combinations. The measuring instrument is driven by the flexible drive system, loaded by the servo electric cylinder, and the linear rotary motion shafting transmits the load and rotation. The measuring system can monitor the bearing speed, load and deformation of the measured bearing in real time, so as to calculate the dynamic of the measured bearing. stiffness. The working principle of the measuring instrument, the structure and working mode of key systems such as the measuring main body, flexible drive system, loading system, etc. are expounded, the technical characteristics of the measuring instrument are introduced, and the reliability of the measuring instrument is verified by experiments.