Considering that the effects of drought on plant photosynthetic parameters have not been quantitatively described in plant mechanistical photosynthesis models, the seedlings of Quercus mongolica from northeast China were used as a case study to analyze the responses of its photosynthetic parameters to different soil water contents. The results showed the relationship between the maximum net leaf photosynthesis rate (Pmax) of Quercus mongolica and soil moisture could be expressed as a quadratic curve (P<0.01), and Pmax reached a maximum when soil volume water content was 35.45% close to field water holding capacity. The maximum rate of carboxylation(Vcmax), the maximum potential rate of electron transport(Jmax) and triose phosphate utilization rate(TPU) of Quercus mongolica also had quadratic relationships with soil water content (P<0.01).Namely，Vcmax、Jmax 、TPU had similar response curves to soil water, but had different optimal soil water contents .This function based on the temperature and water responses of plant photosynthetic parameters provides the researchers with the parameters and methodology for understanding and simulating the responses of plant photosynthetic parameters to drought stress.