Abstract: In order to know the effect of phosphorus fertilizer adding in loess plateau agriculture, the leaf gas exchange parameters, instantaneous water use efficiency (WUEi), leaf water potential and yield were measured under three phosphorus adding (P₀, P₉₀ and P₁₈₀) on the Loess Plateau in Shaanxi Province. And the relationships among them were also analyzed. The results showed that, the photosynthetic performance was enhanced with the adding of phosphorus, and there were significant differences among the treatments. The adding of phosphorus could improve the WUEi, and the leaf water potential was also improved. The highest yield was the treatment P₁₈₀, 4550 kg/hm². There were significant correlations between gas exchange parameters and WUEi, leaf water potential, yield, while the correlation coefficients between photosynthetic performance and leaf water potential, yield were higher than that between photosynthetic performance and WUEi. There was also significant correlation between WUEi and yield.