The work aims to make up the shortcomings of the single tannic inhibitor through modification, so as to improve its corrosion-inhibition efficiency on the iron relics. The inhibition efficiency of the tannic acid and its modified inhibitor on five kinds of simulated iron relic samples was compared and evaluated through the weight loss experiment, SEM-EDS and potentiodynamic polarization method. These inhibitors were applied for tests on the iron relics. The modified tannic inhibitor had excellent inhibition effect to the low sulfur cast iron, slightly lower for the high sulfur cast iron in the corrosion system of 3.5% NaCl aqueous solution. The inhibition film layer was uneven and unstable after inhibited by tannic acid, but homogeneous and stable after inhibited by modified tannic inhibitor. The passivation of the samples occurred due to the improved formula of tannic acid containing sodium silicate, zinc dihydrogen phosphate and ethanolamine. Its corrosion current was greatly lower than those of the non-inhibitor solution and tannic acid. The practical application results showed that the iron relics treated with modified tannic inhibitor had better corrosion resistance than those treated with tannic acid. The modified tannic inhibitor has solved the problem that the acidity of tannic acid potentially impacts the iron relics, overcome the shortage of incomplete and unstable thin film made by the tannic acid, and of changing the appearance color of cultural relics, and greatly improved the inhibition efficiency.