A mannich-base inhibitor for decalcification, a preparation method and application thereof are provided. The inhibitor comprises 10-80% mannich-base component calculated in the total weight percent of the inhibitor, while the rest is at least one compound selected from imidazoline inhibitor with molecular weight between 110 and 750, and alkynyloxy amine inhibitor. The mannich-base inhibitor component is prepared through mannich reaction with 1 mol organic polyamine containing three or more primary amine bases and/or secondary amine bases, 3-7 mol ketones, and 3-7 mol aldehydes. The inhibitor which can be effectively compounded and cooperated with oil demulsifying agent and oil decalcifying agent, have the advantages of stable property, strong absorbability, high film strength and film density with its inhibition rate exceeding 90%. The inhibitor is especially adapted for inhibiting the steel corrosion caused by the mixed medium of salt, acid and water from the desalination and dehydration apparatus of oil refinery below 160° C.