Based on potted experimental data conducted at automatization greenhouse in water-saving park of Hohai university, influence of space-time deficit irrigation on physiological and gronomic features of hot pepper was studied, and the influence mechanism of space-time deficit irrigation on the water use efficiency (WUE) of leaf level of hot pepper was analyzed. The experimental results showed that space-time deficit irrigation had a certain inhibition on the growth of height, leaf area, and other vegetative indexes. However, the moderate space-time deficit irrigation could restrain transpiration rate enormously, but make photosynthesis rate fall slowly. Therefore, WUE of leaf level is considerably improved because the photosynthesis has the compensating effect after rewatering.