Abstract: Aimed to examine the mechanism of nitrogen forms affecting water use efficiency (WUE), a hydroponic experiment was conducted to study the responses of leaf photosynthetic gas exchange parameters to vapor pressure deficit (VPD) in wheat. Results indicated that ammonium nitrogen decreased leaf area and root length, while increased instantaneous and long-term WUE significantly. The responses of photosynthetic gas exchange parameters to VPD were inconsistent under different N forms. With the increase of VPD, net photosynthetic rate (Pn) was kept consistent with the presence of nitrate nitrogen, while it decreased with the presence of the other two N forms. A declination of stomatal conductance (Gs) of all treatments with the increasing VPD was observed, with a more remarkable decrease for the treatments of nitrate nitrogen and ammonium nitrate than ammonium nitrogen treatment. Intercellular CO2 concentration of (Ci) under both nitrate nitrogen and ammonium nitrate treatment decreased with the increasing of VPD while it keep constant under ammonium nitrogen treatment. Transpiration rate (Tr) was quadratically related with VPD, and leaf water potential did not change with VPD, indicating a feed forward mechanism of stomatal response to VPD. Instantaneous WUE decreased with the increasing of VPD for all treatments. The closure of stomata with the presence of ammonium nitrogen under high VPD might be one domain reason resulting in a higher WUE.