<p>In this study, a method for improving the torsional strength of hydraulic expansion-assembled camshafts via microstructural improvement is proposed. First, mandrels with different grain sizes were prepared via annealing. Then hydraulic expansion tests were conducted on the mandrel and cam, and the torsional strength of the camshaft after assembly was measured. Next, finite element analysis was performed using the ABAQUS software package, and a hydraulic expansion model was constructed to simulate the filling process of the assembled camshaft. Subsequently, changes in the contact area and pressure for different grain sizes were obtained, and the torsional strength of the assembled camshaft was determined. The results showed that the change in grain size affected the tensile strength, yield strength, and elongation of the mandrel, which improved the contact state (e.g., contact area and pressure) between the cam and mandrel during the hydraulic expansion process, thereby improving the torsional strength of the assembled camshaft. Moreover, improving the mandrel’s microstructure improved the torsional strength of the camshaft. The proposed method could be used to enhance the reliability of hydraulic expansion-assembled camshafts and promote their application and development.</p>