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Sugar beet is a characteristic cash crop in Inner Mongolia, and water and fertilizer is an important management measure in sugar beet planting. The interaction between water and nitrogen is of practical significance in regulating the growth of sugar beet. The coordinated relationship between water and fertilizer can achieve the purpose of water saving, fertilizer saving, cost saving and efficiency increasing. The cultivation method of drip irrigation under plastic mulch was adopted in this study. The effects of water nitrogen supply on water metabolism and photosynthetic performance of sugar beet were studied through two years of field experiments in order to provide theoretical basis for improving the utilization efficiency of water and fertilizer and rational fertilization and irrigation. The results showed that the water nitrogen supply level and its proportion had influence with the water potential, Transpiration rate (Tr), Photosynthetic rate (Pn) and stomatal Conductance (Cond) of sugar beet leaves. And they were significantly positively correlated with the yield of sugar beet. When the water potential of sugar beet leaves was −0.38 ∼ −0.23 MPa in the period of leaf fast growth, root and sugar growth, and −0.45 ∼ −0.38 MPa in the period of sugar accumulation, the yield and sugar content were the highest. Under the coupling condition of water and nitrogen, the irrigation amount of sugar beet was 1350 ∼ 1427 m3·ha−1, and the nitrogen application amount was 150 ∼ 179.22 kg·ha−1. Sugar beet has the highest utilization rate of light energy and yield and sugar content. [ABSTRACT FROM AUTHOR]