Based on the climatic records of 11 meteorological stations in the upper reaches of the Huaihe River from 1959 to 2015, the correlation analysis was conducted using the climatic yield of summer maize and the precipitation sequence in the growth period to determine the key period that precipitation have the greatest influence on summer maize and define it as the key period of precipitation for summer maize. In addition, 21 distribution functions were applied to fit the precipitation series in the key period of precipitation and meanwhile the test of goodness of fit was conducted by K-S and A-D method to create the optimal probability distribution model. Further, the precipitation thresholds R of all levels of drought and flood in summer maize were designed based on the probability of quantitation of precipitation probability, and the rationality of the index was verified. The results show that (1) precipitation in the tassel-mature stage and the first ten days of the tassel-mature stage has significant effect on the yield of summer maize in the upper reaches of Huaihe River; (2) The optimal probability distribution models of precipitation sequences in the critical growth period of summer maize and precipitation were significantly different at each station; (3) The precipitation thresholds of all levels of drought and flood disasters for summer maize in 11 stations were varied in different regions. The Tyson polygon method was used to determine the precipitation thresholds of drought and flood disasters in the key period of precipitation for summer maize in the upper reaches of the Huaihe River, namely severe drought R < 73 mm, drought 73 mm ≤ R < 138 mm, normal condition 138 mm ≤ R < 337 mm, flood 337 mm ≤ R < 475 mm, severe flood R ≥ 475 mm.