Abstract: A corrosion inhibitor for a soft water circulation heating and cooling system, wherein the corrosion inhibitor is pentapentanol core hyperbranched macromolecule functionalized by isothiourea and carboxyl, which has a chemical formula of C5H7O5{COC(CH3)[CH2OCOCH2CH(COOH)SC(NH)NH2]2}5. The corrosion inhibitor provided by the present invention is non-phosphorus, biodegradable, non-toxic, low in cost and has good corrosion inhibition effect. Moreover, it has excellent corrosion inhibition performance for circulating heating and cooling systems using soft water as the water source, and can significantly improve the reuse rate of circulating water. The present invention also provides a preparation method of the corrosion inhibitor, which is simple, easy to obtain raw materials and easy to industrialize.