The invention discloses a scale inhibitor for a gas pump circulating cooling water system for use in an underground coal mine. The scale inhibitor is composed of 10-16g of ethylenediamine tetraacetic acid, 5-8g of sodium humate, 20-27g of polyphosphates, 35-42g of hydroxyethylidene diphosphonic acid, and 10-16ml of water. A preparation process of the scale inhibitor comprises evenly mixing and stirring the proportionally weighed chemical raw materials to obtain the finished product. The preparation process of the scale inhibitor is scientific and rational in raw material mixing, labor-saving and time-saving, and convenient to operate; the scale inhibitor is capable of removing carbonate scale, sulfate scale, rust scale, and the precipitate and sediment like deposits of other mineral materials safely, effectively and quickly, and non-corrosive, and is also capable of forming a protective film on the internal surface of a metal pipe; besides, the scale inhibitor is stable in performance while used in hard water, free of floccule and precipitate, strong in buffer capacity to pH values and long in service life.