<p>The causes of hub fracture of the sun gear in the planetary gear transmission system were analyzed through chemical composition, mechanical properties, macroscopic characteristics, microscopic structure, simulation and experimental methods. Due to the low nickel content in the gear material, the absence of fillet at the root of the internal spline teeth, and the thickness of the thin hub (which led to severe stress concentration), fatigue cracks began to form in the stress-concentrated area of the internal spline teeth root. The fatigue fracture of the sun gear hub is caused by the initiation of fatigue cracks at the root of the spline teeth under cyclic bending and torsional loads and their propagation along the thickness direction of the thin-rimmed hub. The fundamental reasons for its failure are gear materials with inadequate chemical composition, excessive stress concentration and overly thin hubs. Improvement measures for preventing and avoiding fatigue failure of the sun gear were proposed. A&#xa0;simulation analysis model for the strength of the planetary transmission sun gear was established, and fatigue tests before and after structural improvement were carried out, good consistency was achieved with the on-site fatigue crack initiation and propagation path. After the improvement, the fatigue strength and service life of the sun gear were significantly enhanced.</p>