It was found that less energy transfer could occur between the acridine orange and alizarin red（AR） in pH 6.0 solution.An appropriate amount of melamine added could react with AR to form a charge transfer complex，in which melamine is an electron donor because of its rich electronic structure and alizarin red is an electron acceptor.Since the maximum absorption wavelength of the complex and the maximum emission wavelength of acridine orange are about the same，the effective energy transfer could occur between acridine orange and the complex so that the fluorescence quenching of the system is more.Fluorescence quenching degree of the system had a good linear relationship with amount of added melamine.Accordingly，a novel fluorescence quenching method for the determination of melamine was established.Under the optimum conditions，the linear range of melamine was in the range of 3.38×10-7-2.40×10-4 mol?L-1 and the detection limit was 1.02×10-7 mol?L-1.RSD was 1.1%.The method was used to determine melamine in commercial milk and milk powder with recoveries of 97.7%-102%.The method is low cost，simple，fast and accurate.Furthermore，the method mechanism was further explored by UV spectroscopy.