Abstract: NC horizontal milling and boring machine high-speed spindle box bearing mounting holes require higher coaxial degree, conventional machining methods are difficult to ensure accuracy. For a new type of high-speed spindle box developed by a company, an improvement plan is given from two aspects: assembly and processing, and finally the auxiliary support processing method is determined to be the optimal plan, and the specific operation method is provided, and the method is widely used. The processing pass rate has been greatly improved.