Determination of ammonia nitrogen is an important index to evaluate the water quality.The determination results were affected by many factors. In order to improve the laboratory precision and accuracy, some influence factors and some technical details should be paid attention in determination of ammonia nitrogen by the Nessler's reagent spectrophotometric method.The specific data of the best conditions were analyzed,the optimal determination conditions of ammonia nitrogen chromogenic time was 10 ~ 30 minutes , pH value was weak alkaline or neutral,  temperature was 20 ~ 25 ℃.