Abstract: In order to resolve the cylindricity quality problem of crankshaft bore based on a certain type of diesel engine, processing 5M1E was researched, as a result, "machine" and "material" are the main factors, and deformation of workpiece is the root cause by further analysis. Based on the current situation of fixture, and to modify as little as possible, the number and position of fixture pressing bar were researched and optimized, and through the offline measurement and test cutting verification method, it is concluded that when the pressure of fixture pressing bar is 3 MPa, it can reliably press the workpiece and avoid the deformation of workpiece by pressing, thus solving the out-of-tolerance problem of crankshaft bore’s cylindricity machining. What’s more, the research method provides a reference for improving the machining quality of various crankshaft bores.