Abstract: COD digestion tube and COD digestion apparatus were used to replace the standard methods of color-comparison tubes and autoclave sterilizer for digestion. The sampling volume was adjusted and the amounts of alkaline potassium persulfate and sodium hydroxide were reduced. The experimental results showed that after improved the pretreatment method, the correlation coefficient of the standard curve was 0.999 5, the detection limit was 0.05 mg/L, the relative standard deviation was less than 5%, and the sample recovery rate was 98.0%~102%. Compared with the standard methods, the precision and accuracy of the improved method were higher than that of the standard method and the analysis time was shortened, and the process was simplified. No obvious difference occurred in the comparison between the t test of the improved method and standard method, thus can meet the requirements of rapid and accurate determination of total nitrogen in water.