Huanghuaihai Summer Maize Region (HSMR) is one of the most difficult harvesting areas in the world, particularly in terms of mechanical grain harvesting. In this study, a multi-point experiment was therefore conducted in the HSMR to assess the influencing factors for better implementation of mechanical grain harvesting. 1 250 groups of field test datasets were collected, including 20 harvester types and 420 maize varieties, for the grain quality after mechanical harvesting between 2013 and 2019. The harvesting quality of maize grain was determined using the moisture content, breakage rate, impurity rate, and harvest losses from fallen ears and grain. The results show that the moisture content of grain ranged from 11.13% to 44.60%, with an average of 27.38% for the summer maize harvested by a combine harvester. The breakage rate of grain ranged from 1.11% to 33.94%, with an average of 9.29%. The impurity rate ranged from 0 to 12.25%, with an average of 1.68%. The harvest yield loss rate ranged from 0 to 79.82%, with an average of 3.28%. The average moisture content and breakage rate of grain were significantly higher than the national average values. The average moisture contents of grain at harvest were 25.45%, and 25.05% in 2018 and 2019, respectively, whereas, the average breakage rates of grain were 9.07% and 7.88%, respectively. Although the average breakage rate of grain in the HSMR was still higher than the Chinese national standard (5%), the harvest quality had significantly improved since 2013. There was a quadratic relationship between the breakage rate and moisture content of grain at harvest. The minimum breakage rate occurred at the grain moisture content of 21.08%. A high breakage rate of grain was currently the main quality issue to restrict the application of mechanical harvesting in this region, due mostly to the high moisture content of grain at harvest. The average impurity rate met the national standard of 3%, whereas, the average harvest yield loss rate met the national standard of 5%. Nevertheless, the impurity rate of more than 3% was 17.07% of samples, and the harvest yield loss rate of more than 5% was 19.05% of samples. The grain loss accounted for 28.70% of the total loss, and the ear loss accounted for 71.30%. The harvest loss from fallen ears contributed to the main part of harvest loss. There was a large difference in the number of falling ears in the experimental fields, with a variation coefficient of 235.22%. The proportion of maize varieties suitable for mechanical harvesting tended to increase over the study period, indicating better agreement with the continuous decrease in the grain moisture content, breakage rate, impurity rate, and harvest loss rate. Additionally, the yield remained unchanged over the study period. The grain moisture content and breakage rate clearly represented the heat resources with a gradual decrease from south to north, indicating the highest in the north, the second-highest in the middle, and the lowest in the southern region. Consequently, the optimal selection varieties with early maturity and fast dehydration can contribute to reducing the grain moisture content at harvest under the regional climate in the HSMR. Alternatively, an appropriate maize grain harvesting machine with a low grain breakage rate can also be expected to enhance the harvest quality, with emphasis on the high crushing-resistant maize varieties, cultivation technology, and operating procedures of a harvester, even harvesting at the appropriate time.