The work aims to study the causes for the tooth breaking of 17CrNiMo6 gear shaft in the high-speed work of the commonly used reducer and the improvement measures. The causes for the tooth breaking of 17CrNiMo6 gear shaft were studied and analyzed by means of chemical composition analysis, hardness test, microstructure analysis and other methods. The improvement measures were proposed according to the causes for tooth breaking. The results showed that the factors such as the fatigue fracture, the improper preparation of heat treatment process and the stress concentration caused by nonmetallic debris that exceeded the standard were the main causes for the tooth breaking of 17CrNiMo6 gear shaft. By improving the heat treatment process, optimizing the structure design, improving the processing accuracy and other measures, the tooth breaking of 17CrNiMo6 gear shaft can be improved.