The rigidity of the principal axis of a CNC vertical boring-milling machine was analyzed, and the belt tension and end deformation were also calculated. A project was proposed to improve the structure of the principal axis for finish machining. After the discharge belt pulley was applied on the principal axis, the force transmission and the rigidity of principal axle system were improved considerably. The design and process means were proposed for the three added parts of the improved structure. The improved structure is simplified, secure and low-cost. The machining precision was increased, and the fixing was also become convenient.