Abstract: Nitrate nitrogen can be reduced to nitrite nitrogen by cadmium column, and nitrite nitrogen produced can react with a acidic color reagent composed of sulfanilamide and N-(1-naphthyl)-ethylene diamine in neutral medium to form a rose-red colored diazo compound having its absorption maximum at 550 nm. Linear relationship between values of absorbane of the colored diazo compound and the sum of mass concentration of nitrate nitrogen and nitrite nitrogen was found within 3.00 mg·L^-1. Base on this phenomenon, continuous flow injection method was applied to the determination of nitrate nitrogen in surface water and seawater. Sixty samples could be analyzed in an hour. Imidazole solution (0.075 mol·L^-1, pH 7.50±0.05) was used as buffer in the determination. Granular cadmium with granularity of 0.3 to 0.8 mm packed in a coil tubing to an effective length of 10 cm was used as cadmium column, and it should be activated in the following manner before use:dipping successively in 2 mol·L^-1 HCl solution for 1 min, in 0.005 mol·L^-1 CuSO₄ solution for 2 min and in 2 mol·L^-1 HCl solution again for 5 min. Detection limit (3s) of the method found was 0.000 1 mg·L^-1. Two CRMs were analyzed by the method, giving results in consistency with the certified values. Water samples were analyzed by the method and used as matrixes for recovery by standard addition method, the results obtained were in the range of 99.0%-102%, and values of RSDs (n=6) found were less than 2.5%.