To investigate the nucleation behavior of ammonium sulphate, commonly used as a fertilizer, during cooling crystallization, polythermal methods have been used to determine the metastable zone widths (MSZWs) of ammonium sulphate in aqueous solution from the saturation temperature at 288.15 to 317.15 K under cooling rates of 6.0 to 30.0 K per hour. MSZWs increased with the increasing cooling rate and the saturation temperature. Based on classical nucleation theory, the nucleation potential model linked the MSZWs to the critical nucleation potential and pre-exponential factor. The MSZWs from one saturation temperature or with one cooling rate were used to predict the MSZWs with other saturation temperatures or with other cooling rates, by the nucleation potential model. The estimated MSZWs were compared with the experimental values. Further, according to the characteristics of the electrolyte solution, the supersaturation term in the nucleation potential model was modified by the activity coefficient. The estimated MSZWs showed improved accuracies and a tendency to be positively correlated with both the supersaturation temperature and cooling rate, which was consistent with the experimentally measured MSZWs.