According to GB 11894-1989, when the concentration of the total nitrogen in surface water is determined, the absorbance of the blank test is rather high. During the tests, the factors, such as the purity of oxidant, the deposited time of oxidation solution, the stability of experimental apparatus, the purity of distilled water, experi-mental environment, etc. bring negative influences on the experimental results. Through a series of contrast tests, the water in the tests, reagent, vessels, autoclave and ultraviolet visible spectrophotometry which have influences on the tests have been examined systematically one by one. The main causes for problems have been found out and adjustments have been made.