The machining quality of the casing is very important to the manufacturing and performance of the aeroengine. Due to the split structure, the machining process of the split casing is complicated and the machining quality is difficult to control. In this work, the two optimization indexes of high-precision and high-efficiency are taken as the focus to study the processing flow of the bypass duct split casing. Combined with the material characteristics and structural characteristics, the processing difficulties of the titanium alloy split casing are summarized and analyzed. The process scheme is designed for the key processing difficulties, including the selection of high-precision processing equipment, the selection of tooling fixtures in each process, the selection of tools and the setting of process parameters. On this basis, the milling process of the casing outer surface with complex features is focused on, and the processing area is divided to complete the determination of different milling methods and CNC tool paths. Finally, the cutting process was simulated and a complete and detailed machining process was obtained. Through the actual parts processing verification, the optimized process has improved the processing accuracy and processing efficiency. In this work, the research is carried out from the aspects of process route and cutting parameters, which effectively reduces the machining deformation of the parts, obtains the precise and efficient machining method of the bypass duct split casing processing, and provides some experience for the processing of large annular thin-walled parts.