A pot experiment was conducted to study the coupling effects of water and fertilizer on diurnal variation of stomatal conductance (Gs), intercellular CO₂ concentration (Ci), the net photosynthesis rate (Pn), the transpiration rate (Tr), water use efficiency (WUE), and the vapor deficit at the leaf surface (V_pdl) of the Zanthoxylum bungeanum Maxim 'hanyuan' seedling leaf. We also analyzed the relationships between photosynthesis properties and soil field water capacity (FWC), fertilization amount (including application of NPK, 1/2NPK and non-fertilizer, and the NPK contained 150 kg N/hm² of urea, 60 kg P₂O₅/hm² of calcium superphosphate, and 150 kg K₂O/hm² of potassium sulfate), and environmental factors. In all treatments, the diurnal variation of leaf Gs, Pn, Tr, and V_pdl manifested itself as single-peak curves, with the peaks occurring approximately at 10:00-12:00, 10:00-12:00, 14:00, and 14:00, respectively, without a midday depression. Leaf Ci had a minimum value approximately at 10:00-12:00. The diurnal variation of leaf WUE looked like a double-peak curve, where the first and second peak occurred approximately at 10:00 and 16:00, respectively, but the second peak's value was lower than the first one's. The peak values of leaf Pn occurred approximately at 12:00 with the NPK+50%FWC and 1/2NPK+50%FWC treatments, but it occurred approximately at 10:00 with the other treatments. The mean value of leaf Gs, Pn, Tr, and WUE increased with the fertilization level, whereas the mean value of leaf Ci and V_pdl decreased with the increase of the fertilization level. The variation patterns of the photosynthetic properties depended on the increase in soil water content. The mean value of leaf Gs, Pn, and Tr generally increased until reaching the maximum values and then declined; the mean value of leaf Ci and WUE generally increased; whereas the mean value of leaf V_pdl decreased until reaching a minimum value and then increased. To improve plant growth and attain high leaf Pn and WUE, people should keep soil water content under 35.9-46.7% FWC. Leaf Pn significantly and positively correlated with basal radium (D), seedling height (H), D²H, chlorophyll content, and the chlorophyll a/chlorophyll b ratio. Leaf Gs, Pn, and Tr significantly and positively correlated with photosynthetic active radiation (PAR), and leaf Tr correlated better with air temperature than with other environmental factors. The optimal PAR for leaf Pn was 1263.6 μmol m^-2 s^-1. Our results suggest that appropriate soil water content and fertilizer application prolong the period when leaf Pn reaches the peak value, thereby increasing leaf Pn and WUE and promoting seedling growth. PAR seems to be the key environmental factor affecting leaf Gs and Pn, whereas air temperature is the most important environmental factor affecting leaf Tr.