<p>In order to enhance the service life of the laser-welded gear shaft of a specific type of differential, a fatigue life prediction and optimization design method based on energy is proposed. Initially, energy-life curves are obtained through monotonic tensile and fatigue tests on the laser-welded joints. Subsequently, a finite element model of the differential gear shaft is established, and by applying a moving heat source through programming, the welding residual stress is acquired. Based on this, a multi-load-step nonlinear finite element analysis is conducted on the differential gear shaft to obtain stress-strain responses under various working conditions. The strain energy density method is then applied to assess the fatigue life of the differential gear shaft. Based on suggested method, it is calculated that the fatigue life at the dangerous position of the weld seam is 4765 cycles, while the minimum lifetime of another dangerous point is 4752 cycles. The results indicate that there is a fatigue life deficiency at the weld seams of the differential gear shaft. Therefore, in order to improve the fatigue resistance of the differential gear shaft structure, the fatigue life at the dangerous points is taken as the optimization target, and the geometric dimensions near the dangerous points are used as design variables. An approximate model is constructed using the response surface method, and finally, and fatigue optimization design is performed based on a multi-island genetic algorithm. After optimization, the fatigue life is significantly improved by 3.5 times.</p>