Abstract: The critical nitrogen concentration dilution curve of lettuce was constructed,through the hydroponic cultivation experiments with four different levels of nitrogen treatments(T1 0.00 mmol·L^-1,T2 3.25 mmol·L^-1,T3 6.50 mmol·L^-1,T4 9.25 mmol·L^-1).On this basis,the nitrogen uptake,nitrogen deficiency and nitrogen nutrition index model were obtained,in order to diagnose the nitrogen nutrition more precisely.The results showed that there was a power function relationship betweenplant aboveground biomass and nitrogen concentration（R2=0.903）.Then the highest and lowest nitrogen concentration dilution curve model were derived,and the coefficients of determination were R2=0.891 and R2=0.906,respectively.The nitrogen uptake,nitrogen deficiency and nitrogen nutrition index model of hydroponic lettuce had the same nitrogen nutrition diagnosis results,and the optimum applications of nitrogen in the early and later period were 6.50 mmol·L-1 and between3.25-6.50 mmol· L-1,respectively.This study provided a theoretical basis for the precise management of nitrogen in hydroponic lettuce.