The content of sugar is influenced by sucrose synthase (SS) activity in roots. The effects of nitrogen level in the amino- nitrate ratio on SS activity of leaves and roots, roots yield and sugar content in sugar beet were studied in the field experiment by nutrient solution culture. The results showed that SS activity in leaves was lower than that in roots. With nitrogen level increasing, SS decomposition activity enhanced, and synthesis activity reduced. SS activity was regulated by different nitrogen forms and the ratio of NO3– and NH4+. SS synthesis activity was enhanced as NH4+ increasing when NO3–: NH4+≥1, and it decreased as increasing NH4+ when NO3–: NH4+≤1, and it was the highest when NO3–: NH4+=1. SS decomposition activity was enhanced as NO3– increasing. Sucrose content in root was lowed as nitrogen level increasing, but it was enhanced as NH4+ increasing in the same nitrogen level. Root and sugar yield were the highest in the medium nitrogen level and NO3–: NH4+=1. The result in field experiment corresponded with that in the nutrient fluid culture. It provides a basis for using reasonably nitrogen fertilizer in sugar beet production. The content of sugar is influenced by sucrose synthase (SS) activity in roots. The effects of nitrogen level in the amino- nitrate ratio on SS activity of leaves and roots, roots yield and sugar content in sugar beet were studied in the field experiment by The results showed that SS activity was leaves was lower than that in roots. With nitrogen level increasing, SS decomposition activity enhanced, and synthesis activity reduced. SS activity was regulated by different nitrogen forms and the ratio of NO3- and NH4 + . SS synthesis activity was enhanced as NH4 + increasing when NO3-: NH4 + ≥1, and it decreased as increasing NH4 + when NO3-: NH4 + ≤1, and it was the highest when NO3-: NH4 + = 1. SS decomposition activity was enhanced as NO3-increasing. Sucrose content in root was lowed as nitrogen level increasing, but but was was as as NH4 + increasing in the same nitrogen level. Root and sugar yield were the highest in the medium nitrogen level and NO3-: N H4 + = 1. The result in field experiment corresponded with that in the nutrient fluid culture. It provides a basis for using reasonably nitrogen fertilizer in sugar beet production.