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Efficiently obtaining the concentration of nitrate N (NO-3N) in hydroponic nutrient solution is essential for crop growth. However, it is difficult to realize on-line detection due to the separation of electrode calibration and ion detection. In order to obtain the concentration of NO-3N promptly, conveniently and accurately, an online calibrating and measuring system was established based on dualsensor redundancy control method and nitrate ionselective electrode (ISE). When calibrating, screw nuts were developed for driving ISEs to move in two degrees of freedom, for the purpose of rinsing electrodes and getting standard voltages. Additionally, unknown parameters of the linear function between collected voltages and logarithms of the ion concentrations were recognized under the least squares fitting method. When measuring, a dualsensor redundancy control method was applied to carry out periodically calibrating and rinsing, as well as online measurement based on the principle of maximum error. To test and verify the accuracy of this system, a no-load test and a production test were carried out. The former was compared the online detection concentrations with the actual values of NO-3N in the 14 single-ion (KNO3) solutions, and the latter continuously monitored the concentrations of NO-3N for 12d in the process of lettuce production. The results showed that the dualcensor redundancy control method was reliable. The on-line detection measured by this system and the laboratory detection measured by ion chromatography showed an extremely significant linear positive correlation （P<0.01), and the mean of average relative error was merely 5.64%, and the mean of average absolute error was 1.172×10-5. Therefore, the system could realize the online detection of NO-3N under the continual consumption of nutrient solution, which not only had strongly relation to those determined by laboratory instrument, but also solved the problems of timeconsuming and laborintensive about manual calibration.