Abstract: The rates of maize grain yield loss, grain crushing, and impurity during mechanized grain harvest in China are high. To reduce grain yield loss, the effects of soil fertility improvement on mechanized grain harvest quality were investigated to provide a theoretical basis for optimizing tillage and straw returning measures. Maize cultivars 'XY696' and 'XM6' were planted at high and low densities under different soil fertilities: low fertility (with tillage and straw returning measures of strip cultivation and no-tillage), medium fertility (with subsoiling and deep tillage), and high fertility (with straw incorporation, subsoiling, and straw incorporation with deep tillage). The farm rotary tillage (with much lower fertility) served as the control treatment. The following mechanized grain harvest quality indicators were measured: ear height uniformity, lodging rate, dehydration rate, and grain moisture content, as well as the yield and yield components. The results showed that soil fertility, maize cultivar, and planting density significantly (P < 0.05) affected the quality indexes of mechanized grain harvest, maize morphology characteristics, grain dehydration, and maize yield. Soil fertility improvement reduced grain yield loss during maize mechanized grain harvest, whereas the grain crushing and impurity rates did not change with soil fertility improvement. Under high planting density, yield loss decreased by 12.55-15.70 percentage for each fertility unit. Yield loss increased with increasing planting density, and the loss rate of 'XY696' was more than that of 'XM6'. Soil fertility improvement led to an increase in ear height uniformity (5.35-9.69), reduced maize lodging (5.44-9.75 percentage), and increased the grain dehydration rate (0.048-0.090%·d^-1). Optimization of these indexes may explain the reduction in yield loss at high fertility. Increased planting density reduced ear height uniformity and increased the maize lodging and grain dehydration rates. Soil fertility improvement effectively alleviated the negative impacts of densification. 'XY696' had lower ear height uniformity, higher lodging, and slower dehydration compared to 'XM6', which led to higher grain loss for 'XY696'. Soil fertility improvement increased the ear numbers per unit area, grain numbers per ear, and 1000-grain weight, ultimately increasing yield by 1878.5-2544.4 kg·hm^-2 for each fertility unit increase. The increase in maize grain yield was due to a reduction in grain yield loss during mechanized maize grain harvest. The number of ears per unit area increased, whereas the grain number per ear and the 1000-grain weight decreased when the planting density increased. Maize grain yield increased when the planting density increased at high fertility levels. Therefore, soil fertility improvement via tillage and straw returning can increase maize yield and reduce yield loss during mechanized grain harvest in Inner Mongolia. Under high soil fertility, a reasonable planting density increase can improve the yield and harvest quality and decrease the grain moisture content. Reduced mechanized grain loss can be achieved by selecting maize cultivars with high lodging resistance, high ear height uniformity, and a fast dehydration rate.