Abstract: The technology of focused beam reflectance measurement（FBRM） was used to test the crystallization metastable zone width of the ammonium dihydrogen phosphate solution.By determination of the nucleation temperature of ammonium di- hydrogen phosphate solution under different cooling rates，the ammonium dihydrogen phosphate′s metastable zone width data were obtained.Results showed that the supersaturated concentration and metastable zone width of ammonium dihydrogen phosphate solution increased along with the decrease of temperature under the same cooling rate.Under different cooling rates，ammonium dihydrogen phosphate metastable zone width increased with the increase of cooling rate，and the largest metastable zone width was generally at the cooling rate of 1.0 ℃/min.When the cooling rates were 0.2 ℃/min and 0.1 ℃/min， the width of metastable zone remained constant.

Key words: focused beam reflectance measurement, ammonium dihydrogen phosphate, cooling rate, metastable zone width