<p>Halogenated hydantoin is a fungicide and disinfectant employed in industrial and domestic environments, which divides 1,3-dichloro-5,5-dimethyl hydantoin, 1,3-dibromo-5,5-dimethyl hydantoin, and 1-bromo-3-chloro-5,5-dimethyl hydantoin based on different product types. Nevertheless, several thermal runaway incidents have occurred during applications, resulting in severe consequences. Consequently, it is of paramount importance to conduct a comprehensive study of their thermal decomposition characteristics. This paper employed a thermogravimetric analyzer (TG) and differential scanning calorimetry (DSC) to investigate halogenated hydantoin exothermic and evaporative behavior under nonadiabatic conditions. An accelerating rate calorimeter (ARC) also examined their thermal behavior in pure and mixed aqueous environments. The results indicated that 1,3-dichloro-5,5-dimethyl hydantoin exhibited the most prominent risk under nonadiabatic and adiabatic conditions, and 1,3-dibromo-5,5-dimethyl hydantoin demonstrated superior thermal safety performance. Adding water significantly increases the thermal hazard for all three substances. For DCDMH, 0.5&#xa0;g of water lowers the temperature corresponding to the maximum reaction rate reached in a time of 24&#xa0;h (TD24) by 24.09&#xa0;°C. Therefore, contact between halogenated hydantoins and water should be strictly avoided in production. Moreover, it was revealed that NO₂, cyanide, and chloromethane were formed during thermal decomposition of the halogenated hydantoin, necessitating enhanced toxic gas monitoring. Reducing storage specifications through revised standards was prudent to ensure optimal safety in regions with elevated annual mean temperatures, and plastic drums were safer packaging materials. These findings offer essential guidance for safely handling halogenated hydantoins in production, transportation, and storage, thereby enhancing the safety management of hazardous chemicals.</p>