Objective To explore the reliability and applicability of continuous flow injection analyzer for determination of ammonia nitrogen concentration in water. Methods Samples were analyzed automatically on line by flow injection analysis technology, and the test reaction was based on the improved Berthelot reaction, in which ammonia nitrogen in water was chlorinated to form monochloramine, and the monochloramine reacts with salicylate to form 5-amino salicylate. The green complex was produced through oxidation and oxidation coupling reaction, and was determined at the wavelength of 660nm. Results Under the experimental conditions, a mmonianitrogen has a good linear relationship between 0 and 1.00mg/L. The correlation coefficient was 0.9998 and the detection limit was 6.33μg/L.The mean relative standard deviation was <2％, and the recovery rate was 96.2%-99.6%.All the certified standard substances were measured within the specified range, and relative error was <2％. 20 water samples was measured simultaneously by flow injection analysis and Nessler's reagent colorimetric method, the results showed that there was no statistic difference between the two methods(t=0.988,P=0.336). Conclusion The method of measuring the ammonia nitrogen in water by flow injection analyzer was proved to be sensitive and accurate, which greatly reduces the measurement time, and is especially suitable for analysis of large-scale water samples.