Because leaves are the chief organs of photosynthesis, changes in leaf water content affect photosynthesis. Leaf water content can directly reflect crop growth and development, and indirectly reflect the degree of atmospheric drying, ability of soil to supply water, and drought tolerance of crops. Until now, soil water content and its effects on plant photosynthetic parameters have been investigated in numerous studies, but the influence of leaf water content on photosynthesis has not yet been reported. Estimation of this influence is essential for accurate simulation of photosynthesis. In this study, summer maize from north China, which suffers from frequent droughts, was used to determine the relationship of leaf water content with net photosynthetic rate (P_n) and soil water content, by performing water manipulation experiments using 3-leaf stage plants. On the basis of the average monthly natural precipitation for July, over 30 years from 1981 to 2010, in Baoding, Hebei Province, six watering treatments (7%, 20%, 40%, 60%, 80% and 100%) were used, with three replicates per treatment. The watering treatments with disposable irrigation were used to simulate the effects of consecutive droughts of different intensities on the photosynthetic characteristics of summer maize. The plants were not watered after July 2. The soil water content, fresh weight and dry weight of leaves, and net photosynthetic rate (on the same leaf on the same plant) were measured under sunny weather conditions every 1 to 2 weeks. The different watering intensities strongly affected the crop index of summer maize. The results indicated that the leaf water content of summer maize significantly affected the net photosynthetic rate, the two factors showed a quadratic curve relation under all weather conditions or developmental stages of plant growth, but with different curve coefficients. On an average, P_n reduced to zero when the leaf water content reached approximately 70.3%, this was considered as an important threshold in the photosynthetic physiological activity of leaves. It can provide the basis for dynamic monitoring of large-scale drought development and accurate assessment of drought degree in crops. Moreover, leaf water content showed a non-rectangular hyperbolic relationship with soil moisture, which differs from the linear relationship reported in the previous studies, because of the narrow water gradient. The leaf water content increased with increasing relative soil water content and then tended to plateau, which is called the leaf water holding capacity. According to the Michaelis-Menten equation, the maximum leaf water content was estimated to be 85.14%, using the double reciprocal Lineweaver-Burk plot. The leaf water holding capacity and the critical leaf water content are different for different crops; however, a consistency in the research methods and laws of response may provide a reference for understanding the effect of changes in leaf water content on photosynthesis. These results could contribute to objective identification of the occurrence and development of drought events in summer maize.