Abstract: The determination of ammonia content in fish could be used in the freshness test of fish. Under alkaline conditions, NH3 reacted with phenol-sodium hypochlorite at normal temperature under the catalysis of sodium nitroferricyanide to produce stable blue chemical compound. The maximum absorption wavelength was 620 nm,the best color time was 20 min and the best color temperature was 37 ℃, relative standard deviation was 1.5% （n=10）. And the recovery rate was 96.7%-101.5%. This method was simple, rapid and accurate.