Abstract: As a key parameter for determining crop water consumption, accurate estimation of the crop coefficient (K_c) is important for irrigation scheduling. K_c is influenced by changes in production and meteorological conditions. This study analyzed K_c changes from 1980 to 2018 for summer maize under sufficient water supply based on long-term field experiments at the Luancheng Agro-Ecosystem Experimental Station of the Chinese Academy of Sciences. Using data of the three most recent seasons from 2019 to 2021 for maize under normal and water-deficit conditions, the calibration coefficient of K_c by soil water content was developed and tested. The results showed that the reference crop evapotranspiration (ET_o) was stable from 1980 to 2018; however, seasonal fluctuations were observed. The actual evapotranspiration (ET_c) of summer maize under adequate water supply conditions has substantially changed over recent years. The multiyear average K_c for maize was 0.91, and the interseasonal variation coefficient was 12.36%. K_c is affected by crop yield and atmospheric conditions. An increase in yield was accompanied by an increase in the ET_c of summer maize. ET_o was mainly affected by average wind speed and sunshine hours. Our analysis revealed that K_c is determined by ET_o and ET_c and is greatly affected by the ET_c of summer maize. For the three most recent seasons, the difference in irrigation quantity was the main factor causing differences in K_c among the different treatments in the same season. K_c adjustment using different methods considering the soil water content could be used to estimate ET_c. Incorporating the root length distribution factor into the soil water status for K_c adjustment provided a better estimate of ET_c using the crop coefficient method. Therefore, K_c can be adjusted according to the root-zone soil moisture status to determine the actual crop water consumption.