Abstract: Glycine max (L.) Merr. in northern China were selected as the experimental materials, with photosynthetic limitation as the primary focus of this study. A soil water-nitrogen interaction experiment was designed to explore the relationship between photosynthetic limitation and leaf instantaneous (WUE_ins) and intrinsic water use efficiency (WUE_int). Results showed that: (1) CO₂ diffusion limitations, including mesophyll limitation (l_m) and stomatal limitation (l_sc), gradually increased with water stress, while biochemical limitation (l_b) gradually decreased. (2) The interaction between water and nitrogen reduced l_m and l_sc, while l_b significantly increased. (3) l_m and l_b were negatively and positively correlated with WUE_ins and WUE_int, respectively (P<0.05), while l_sc showed no significant correlation with either. (4) Among the limiting factors, l_m contributed most to soybean photosynthetic carbon assimilation and leaf water use efficiency, making it the dominant constraint on photosynthetic and water use capacity. These findings reveal the ecological effects of soil drought and atmospheric nitrogen deposition on crop photosynthetic carbon assimilation and their intrinsic relationship with water use capacity.