The invention relates to a green slow release corrosion-resistant descaling agent and a preparation method thereof and belongs to the technical field of descaling agents. According to the preparationmethod, humic acid in peat is extracted; humic acid as a surfactant is dissolved with water to prepare a sodium humate solution; as the humic acid solution is a hydrophilic colloid, when the humic acid solution is added into Al3<+> and La3<+> solutions which are relatively low in pH, coordinate bonds are formed by hydroxyl by virtue of carboxyls on humic acid to crosslink; humic acid gelatinizes and gathers and meanwhile, Al3<+> in the solution hydrolyzes and polycondensing reaction happens, protons generated by hydrolysis of Al3<+> are continuously bonded with carboxyls and hydroxyls on humicacid molecules to promote hydrolysis and polymerization of aluminum salt and intensify gelatinization of humic acid so as to form a compound gel. Impurities needed to be removed are coated effectively to form a slow release degrading effect, so that the descaling property of the compound descaling agent is improved effectively.