Abstract: Taking a typical apple×maize intercropping system in the loess region of Shanxi Pro-vince, China as test object, the current study analyzed the photosynthetic characteristics of maize leaves of ear position at filling stage under different irrigation and fertilization regimes. There were three irrigation levels [low (W₁), 50% field capacity (Fc); medium (W₂), 65% Fc; and high (W₃), 85% Fc], three fertilizer levels {F₁ [N (289 kg·hm^-2) + P₂O₅ (118 kg·hm^-2) + K₂O (118 kg·hm^-2)]; F₂ [N (412.4 kg·hm^-2) + P₂O₅ (168.8 kg·hm^-2) + K₂O (168.8 kg·hm^-2)], F₃ [N (537 kg·hm^-2) + P₂O₅ (219 kg·hm^-2) + K₂O (219 kg·hm^-2)]}, and a control (CK, no irrigation and fertilization throughout the growing season). The nine irrigation and fertilization treatment combinations were W₁F₁, W₂F₁, W₃F₁, W₁F₂, W₂F₂, W₃F₂, W₁F₃, W₂F₃, and W₃F₃, respectively. The results indicated that different water and fertilizer regimes had no significant effect on the variation in diurnal photosynthetic indexes. However, the application of water and fertilizers considerably increased the peak value of net photosynthetic rate (P_n), exten-ded the stomatal opening period, influenced the lowest value of intercellular CO₂ concentration (C_i), and reduced the maximum value of water use efficiency (WUE) during the day. The limiting factors affecting photosynthesis were nonstomatal factors. Stomatal conductance (g_s) and transpiration rate (T_r) were both significantly negatively correlated with the distance from tree line (P<0.01). WUE were significantly positively correlated with the distance from the tree line (P<0.05). g_s decreased by 0.028-0.093 mol·m^-2·s^-1, T_r decreased by 0.56-1.41 mmol·m^-2·s^-1, WUE increased by 0.08-1.00 μmol·mmol^-1, as the average distance from the tree line increased by 1 m. Thus, irrigation and fertilizers significantly increased the mean value of P_n, T_r, and g_s, but decreased the mean value of WUE during the day. The W₃F₁ treatment had the highest mean values of P_n(10.64 μmol·m^-2·s^-1), g_s(0.295 mol·m^-2·s^-1), WUE (3.05 μmol·mmol^-1), but a lower mean value of T_r(4.32 mmol·m^-2·s^-1) compared with the other treatment combinations during the day. When the total irrigation rate was 1300 m³·hm^-2, the total fertilization rate was 525 kg·hm^-2, P_n was at its maximum, and the theoretical maximum value was 10.32 μmol·m^-2·s^-1. Therefore, the W₃F₁ treatment was the irrigation and fertilizer regime that was most likely to improve the photosynthetic efficiency of the apple-maize intercropping system.