Abstract: To solve the problem of high dimensional tolerance requirements for multi-hole position, same column, parallelism, perpendicularity and surface roughness of medium speed diesel engine block, the main bearing hole is determined to be the most important reference for the whole block by analyzing the five elements of man, machine, material, method and ring in the manufacturing process. Due to the limitation of body structure and accessory head size, secondary clamping can only be used. When it is necessary to re-establish the coordinate system of the rotary table, the macro-program is used for one-time positioning, and the four processing coordinate systems around the machine body are established to improve the positioning accuracy. To measure the machining tact of fuel injection camshaft bore and intake and exhaust camshaft bore and the actual elongation of No. 1 accessory head and No. 2 accessory head every 30 min during rough, semi-fine and fine machining, fit the above data with the Five-order Polynomial Least Squares Method, solve the change rule of accessory head elongation with machining time and make dynamic compensation. Based on bearing holes No. 1 and No. 8, the measured deviation values of the multi-way holes are within the technical requirements, and the high-precision control of the multi-way holes of the body of the medium-speed diesel engine is realized.