In order to clarify the relationship between milk line proportion and grain moisture content and grain filling process and provide references for the decision-making of maize harvest period, a field observational experiment was conducted with 9 spring-sown maize varieties in 2017 and 2018 at Yongning County, Yinchuan City, Ningxia Hui Autonomous Region. In this study, the cross-sectional images, grain moisture content, and 100-grain dry weight were measured continuously on the central part of the ear within 20 to 80 days after silking. An image-assisted software tool was designed to measure the milk line proportion of maize grains, accurately obtaining the information of milk line proportion in different varieties or different growth stages. Regression analysis showed that there were slight differences in the variation of milk line proportion, grain moisture content, and filling process between varieties and inter-years, which generally conformed to the same variation regulations. There was a significant linear relationship between milk line proportion and grain moisture content, and the regression equation was y = ?0.2572x + 52.482. The relationship between milk line proportion and grain filling process was fitted to Richards curve significantly, and the regression equation was y = 99.65/ (1+exp(2.45?0.07x))^(1?3.70). The changes of milk line proportion were closely related to grain moisture content and grain filling process within a certain range, which can be used as one of the field evaluation indicators for different kinds of maize harvest.