A method for preparing the water treating agent ployepoxide sulfonic carboxylic acid of biodegradable, with the characteristics being: employing epichlorohydrin and bisulfate with molar ratio being 1:1-1:1.5 as raw material, synthesing intermediate compound under action of phase transition catalyst alcohol isopropylicum with its consumption being 1-20% of that of total reactant and nitrogen, the intermediate compound without separation directly polymerizing with epoxide succinic acid under the catalytic action of rare-earth metal catalyst such as Ba and Sr with its consumption being 0.1-1% of that of total reactant and getting the ployepoxide sulfonic carboxylic acid or its salt solution. |