The selection of the appropriate harvest time and moisture content (MC) is very important for maize (Zea Mays L.) production, and selection is an essential step that balances the yield and comprehensive benefits. In this study, the process and final MC following a grain dry-down of sixteen maize hybrids was observed in 2017 and 2018. The Guggenheim-Anderson-de Boer (GAB) model, which is commonly used to describe the equilibrium moisture content (M_eq) under controlled conditions, was used to evaluate the relationship between the field M_eq and the final MC. The results showed that the MC reached a stable level after a long dry-down period in the field, indicating that an equilibrium was present between the grain and the atmosphere. The MC at the equilibrium state represented two characteristics, a “hysteresis effect” and a “convergence effect”, which were as follows: the change in MC always lagged behind the M_eq in the atmosphere; and the MC fluctuated continuously with the M_eq but could not reach its limit. The time of the “hysteresis effect” was estimated to be approximately 2-4 days. The time to reach equilibrium varied greatly in different maize hybrids, and it was related to the growth period and dry-down characteristics. Moreover, the various climatic types contributed to diverse M_eq values, and the humid climate had a higher M_eq than the dry-cold climate. The distribution of M_eq values and the drying days on which the maize grain moisture content decreased from 30% to 25% (and 20%), guided the threshold for the delayed harvest of maize, as also estimated in various maize ecological areas in China. The optimal harvest time were significantly different in various regions due to different M_eq values. Therefore, we suggest that the estimation of the M_eq could be an important indicator for predicting the grain dry-down and harvest management strategies for a region.