Abstract: The verification and improvement were made on setting ways of the threshold about criticality accident alarm system (CAAS) in national standard GB 15146.9—1994. Source term parameters of different nuclear critical safety accidents in “minimum criticality accident of concern” and shielding properties of various shielding materials were studied. Determination methods of revised threshold and effective operation radius were obtained. The results show that different fission source terms and shielding materials exert great impact on threshold setting of CAAS and it is required to take the contribution of secondary photon into consideration in terms of shielding material research. The formula of threshold after revision provides reference for application of CAAS in real plants.