l bearings for high-speed electric spindle, a method for measuring the dynamic support stiffness of spindles by means of synchronous excitation is proposed. The dynamic support stiffness test principle is derived by studying the angular relationship between the residual vibration displacement and the excitation response displacement. The dynamic stiffness test is carried out on the electric spindle of the model 150SD40Q7, and the simulation results are compared and analyzed. The test and simulation results have the same trend with a small error. The validity of the test method is verified, which provides a reliable method for testing the dynamic stiffness of the spindle.