The invention discloses an inhibitor with sustained release property and a preparation method thereof, and the inhibitor is characterized by comprising the following steps of preparing a polymer emulsion by evenly mixing 10-20% of polymer, 79-89% of solvent and 1% of surfactant according to the percentage of the total weight of the prepared emulsion, further adding 4-9 times of the weight of the polymer emulsion of an inorganic salt type inhibitor, stirring for 2-3 hours at the temperature of 40-50 DEG C, drying an obtained product for 6-8 hours at the temperature of 60-80 DEG C, smashing and then obtaining the inhibitor product with the sustained release property. The preparation method is simple and easy to operate, the obtained product can be sustainedly released during the prevention and control process of coal spontaneous combustion of a mine, thereby prolonging the time of flame retardance; the residual polymer is covered on a coal layer, thereby also playing the roles of oxygen isolation and inhibitation; furthermore, the polymer can utilize waste plastics, and the preparation and the using process can not cause harm to environment, thereby having great application prospect.