Abstract: Abstract: Grain impurity content is one of the most important indexes for the performance of mechanical grain harvest technology in modern maize production. The objective of this study was to explore the relationship between gain impurity rate and cob characteristics, thereby realizing high-quality grain harvest for the better development of mechanical grain harvest in southwest China. A field experiment was also conducted at Zhongjiang County, Sichuan Province from 2018 to 2020. The same combine harvester and operating personnel were used in different harvest date, in order to investigate the variation regularity of impurity components and cob characteristics. The results showed as below: The impurity rate, absolute content of cob in impurity, and cob moisture content decreased significantly, while the cob bending strength increased at first, and then decreased gradually, with the delay of harvest date. The cob was also the main impurity component in different harvest dates, accounting for 32%-79%, and the average was 51.45%. A systematic analysis was made to clarify the correlation between the grain impurity rate, cob bending strength, and moisture content in different harvest time. It was found that there was no correlation between grain impurity rate and cob bending strength, whereas, an exponential relationship between the grain impurity rate and cob moisture content, where the fitting equation was y=0.045 6e0.063 7x (R2=0.774 7**, n=75). For instance, the grain impurity rate was reduced to less than 3% in the mechanical grain harvest, when the moisture content of maize cob was reduced to less than 65.72%. Nevertheless, there was little relationship between the grain impurity rate and cob bending strength at the same harvest dates, or when the cob moisture content was similar. The grain impurity rate in the study area was generally in agreement with the national standard maize production during mechanical grain harvesting under similar conditions. Since the cob was the main impurity component, the cob moisture content was widely expected to well explain the change of grain impurity rate. Consequently, two suggestions can be made to reduce the grain impurity rate during mechanical grain harvesting. One is to select and breed the maize varieties with low moisture content and fast dehydration, and another is to mechanically harvest with low moisture content via delaying harvest time in maize production.