Abstract: According to the customer's demand for high-speed horizontal CNC milling machine, a new set of main transmission structure is designed. The vibration problem of the spindle box in the manufacturing process is analyzed and the improvement method is proposed. The residual unbalance and cutting vibration of the spindle before and after improvement are detected. At the same time, the stiffness of the drive shaft is tested under the maximum torque condition. The comparison shows that the dynamic balance effect and cutting vibration effect of the transmission shaft made of carbon fiber are improved obviously, and the rigidity is enough to meet the limit working conditions of the machine tool.