This invention relates to a zinc phosphate coated superfine aluminium hydroxide compounding fire-retardant agent and its preparation. Superfine aluminium hydroxide powder is scattered in to water with hexa- sodium metaphosphate for making suspension, and then zinc salt solution and phosphate solution are added into suspension simultaneously for coating reaction, at last, the reaction product of coating reaction is filtered, washed, dried, deploymerized and conducted surface reshaping treatment with couplant. This compounding inorganic fire-retardant agent makes the chemical combination of aluminium hydroxide and zinc phosphate into truth, so the coordinated fire-retardant effect of aluminium hydroxide and zinc phosphate can be played at greatest extends to improve the fire-retardant efficiency of compounding inorganic fire-retardant agent obviously.