<p>The hole expansion strengthening is a pivotal technology for enhancing the performance of hole structures. The structural design of the mandrel plays a crucial role in determining the effectiveness of expansion strengthening and the subsequent fatigue performance of the hole structure. To explore the impact of the mandrel structure on hole expansion strengthening, a 3D finite element simulation model was established to analyze the hole expansion processes for both solid mandrel and split mandrel. Additionally, a series of hole expansion strengthening experiments were carried out, along with fatigue performance tests on specimens with holes, to investigate the influence of plastic deformation on the fatigue performance. The research results show that the maximum relative error range between the simulated residual stress values on the hole wall and the experimental data ranges from 5.3% to 9.7%. During the expansion process of the split mandrel, the deformation displacement of the hole wall in the Y-axis direction is 1.72 times that of the solid mandrel, while the deformation displacement in the Z-axis direction is smaller than that of the solid mandrel. Furthermore, when using the split mandrel hole expansion strengthening method with a relative expansion amount of 3%, the average fatigue life of the specimen can be enhanced to 1.52 times that of the specimen unexpanded. The hole expansion strengthening using a split mandrel can increase the degree of plastic deformation in the radial direction of the hole wall material, thereby improving the fatigue performance of the specimen with hole.</p>