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Based on the daily meteorological data of 141 meteorological stations in the Huaihe River Basin from 1961 to 2015, water requirement and its spatiotemporal variation characteristics during the summer maize growth stages were analyzed using the Penman-Monteith (P-M) formula and the crop coefficient method. The crop water deficit index (CWDI) was used as an index of agricultural drought assessment to reveal the spatiotemporal characteristics of drought during the summer maize growth period. To estimate the probability of drought at different growth stages of summer maize in the Huaihe River Basin, the water deficit index sequences were fitted and the optimal probability distribution model was established by using 33 different distribution functions in four major categories. The results demonstrate the following conclusions. (1) In the last 55years, there was an obvious decrease in the water requirement of summer maize during its growth period, and the spatial distribution characteristics indicated higher requirement in the central and northern parts of the basin and lower requirement in the southwestern and southeastern parts. (2) There was no significant trend in the water deficit index of summer maize at different growth stages and except at the jointing-tasseling stage, the water deficit was obvious at all other growth stages, with a more severe deficit in the northern part of the basin compared to the southern part. (3) During the summer maize growth period, the probability of drought occurrence was highest at the sowing-seedling and tasseling-milking stages, and except for the 30-50% probability of extreme drought at the sowing-seedling stage, the probability of different levels of drought were all generally within 20% at all other stages.