Abstract: In order to investigate the relationship between grain filling rate, dehydration rate and mechanized harvest of the summer corns in Huang-Huai-Hai region, we selected 8 varieties (A, B, C, D, E, F, G, H) with large planted areas to measure and analyze the traits of grain filling rate, 100-seed weight, water content and milk-line position. Results showed that when the milk line 90% retreated the grain filling of all the 8 varieties finished and the value of 100-seed weight reached to the highest, the maximum value is 43.02 g, which indicated the positive correlation between the grain filling time and 100-seed weight. However, when the milk line completely disappeared, the 100-seed weight was largely reduced by 8.66%. There was no significant difference between the traits of 8 varieties during the periods of grain weight rising, but in the periods of grain weight falling, the traits of D, E, H were significantly different with the other varieties and dehydration rate of C and A had significant difference with the other six varieties. The water content of grains was negatively correlated with milk-line position. As the milk line 90% retreated the grain water content decreased to <30%. Taken together, the key factor influencing the mechanized harvest of summer corns is harvesting time, rather than the varieties. Moreover, the period of milk-line position 90% retreated is the best time for harvest; if the harvest is too late, the yield will be reduced with varying degrees.