Abstract: Tooth fracture failure phenomenon occurred in the early stage after the large helical gear of a wheel loader driving axle worked for a period of time during its development process. Through macroscopic inspection, fracture analysis, chemical composition analysis, hardness test, metallographic examination and other methods, the reasons of tooth fracture failure of the large helical gear were analyzed. The results show that the small radius of curvature of the root fillet, the small meshing spot and the low hardness of the tooth center resulted in the intensifying of stress concentration at the tooth root, which was the main reason for the early fatigue fracture failure of teeth of the large helical gear. Finally, according to the failure reason of the tooth fracture, the large helical gear was improved correspondingly, and good results were achieved.