Abstract: The spatiotemporal dynamics of heat index of spring maize at different developmental stages of the whole growing season were examined using datasets including daily average air temperature during the past 55 years (1961-2015) at 82 meteorological stations and thephenophases of spring maize from 1981 to 2013 at 50 agrometeorological stations. The results showed that the heat index of spring maize at different developmental stages exhibited significantly increasing trends. The rapidest increase of heat index occurred at the trefoil elongation stage, reaching up to 0.20 a^-1. The slowest increase was at the flowering milky ripening stage, with a value of 0.09 a^-1. The increasing trends of heat index in more than 50% meteorological stations passed the 0.1 significant test at all developmental stages except for the emergencetrefoil stage, and the percentages at both the trefoilelongation stage and the whole growth period reached up to 95%. The heat index of spring maize over the past 55 years displayed spatial distribution patterns, with higher values in the southwestern regions and lower values in the middle and northern regions for both whole growth period and different developmental stages. Moreover, the change rate of heat index for spring maize was faster in the northern regions than that in the southern regions. In terms of spatiotemporal dynamics, the area with smaller heat index during the whole growth period was gradually shrinking, whereas the area with larger heat index was gradually expanding over time.