Abstract: Studying the impact of harvesting date on mechanical grain-harvesting quality is of great importance to determine a suitable harvesting date and promote the mechanical grain harvest technology. This paper describes an experimental study of the effects of harvesting date (31^st July, and 7^th, 13^th, 19^th, 25^th, and 31^st August) on mechanical grain-harvesting quality and the relationship between grain moisture content and mechanical grain-harvesting quality in Sichuan spring maize. The results showed that, with a delay in harvest date, grain moisture content decreased gradually, and the broken-grain rate decreased rapidly at first and then increased slightly. The impurity rate decreased rapidly and tended to be stable, while the ear-loss rate increased significantly and the variation in grain loss rate was not notable. The total grain loss rate was primarily associated with ear loss, which accounted for 76.34% of the total average grain loss. With a delay in harvest date, the differences in broken-grain and impurity rates between varieties decreased gradually while they increased for ear-loss and total grain-loss rates. Grain moisture content was the key factor affecting the quality of mechanical grain harvest, the relationship between the broken-grain rate and grain moisture content could be described by the equation y=0.032 9x²-1.332 8x + 15.529 (R²=0.55^**, n=72). The broken-grain rate was lower than 5% when the moisture content was between 10.76% and 29.76%. The relationship between the impurity rate and moisture content could be described by the equation y=0.031 8e^{0.118 5x} (R²=0.71^**, n=72), and the grain impurity rate was lower than 3% when grain moisture content was lower than 38.37%. The relationship between the ear-loss rate and moisture content could be described by the equation y=2 083.3/x^2.135 (R²=0.68^**). The relationship between the total grain-loss rate and moisture content could be described by the equation y=911.02/x^1.769 (R²=0.68^**), and the total grain-loss rate could be lower than 5% when grain moisture content was higher than 18.96%. A delayed harvest was associated with a decrease in the broken-grain and impurity rates and an increase in the risk of ear-loss and the total grain-loss rate. The optimal grain moisture content for mechanical harvesting in Sichuan spring maize was 18.96%-29.76% and the optimal mechanical-grain harvesting date was between August 7 and August 19 in this study, which was approximately 10-15 days later than the traditional harvest date.