Post-processing is an important technology in transferring information between CAM software and CNC system. At present, the traditional five-axis machine tool cannot satisfy the requirements of high precision and efficiency in the manufacture of complex parts due to its single function. The auxiliary rotary spindle, auxiliary linear axis, and power tool rest are added on the basis of the traditional five-axis machine tool to solve the technical bottleneck problem. Thus, the machine tool with the composite function of turning and milling is established. The control of the motion chain and the numerical control system of the turn-milling composite machine tool is more complex than that of the traditional five-axis machine tool, and the development of the post-processing is also more difficult. This study investigates the development of post-processing of a nine-axis five-linkage turn-milling composite machine tool (PUMASMX2600ST). First, the kinematic chain and coordinate system of the turning and milling function are established, respectively, according to the function division of the machine tool, and the inverse kinematic model is solved by the kinematic coordinate transformation. Second, a special post-processing software with turning and milling functions is developed using Visual Basic language. Finally, a type of blade root milling tool is used as the specimen for simulation and actual cutting verification. Experiments show that the special processor is correct and reasonable, and it can satisfy the requirements of the machine tool.