Abstract: The standard method for the determination of ammonia nitrogen in environmental surface water was not comprehensive by gas phase molecular absorption spectrometry. Therefore, the effects of preservation of water samples, nitrite, Ca²⁺, Mg²⁺, I^- and sulfides on the determination results were investigated. As shown by the results, after collecting the surface water sample, sulfuric acid was added immediately to acidify the water sample (pH<2), and the sample was sealed, which could prolong the storage time of the sample to 7 d, but the determination should be carried out within 24 h. When the nitrite content was high, the interference could not be eliminated in the ammonia nitrogen removal of nitrite function mode by gas phase molecular absorption spectrometry, while nitrate interference for ammonia nitrogen determination could be effectively eliminated by heating and boiling or pre-distillation before analysis of the water sample. Furthermore, no significant interference was observed by Ca²⁺, Mg²⁺, I^- with mass concentration less than 25 times and sulfide with mass concentration less than 10 times in water. The detection limit (3.143s) of this method was 0.02 mg·L^-1. Test for recovery was made by standard addition method, giving results in the range of 94.7%-101%, with RSDs (n=6)of the determined values in the range of 0.70%-4.7%. The standard samples were analyzed by gas phase molecular absorption spectrometry, giving results within the allowable deviation range of the certified values of the standard samples. Compared with the results of Nessler reagent spectrophotometry, gas phase molecular absorption spectrometry with per-distillation had better precision and accuracy.