The invention relates to a butter inhibitor used for an ethylene unit alkaline washing tower. Problems of the background technology are solved, so the butter inhibitor has the performance of oxidation resistance, polymerization inhibition, cleanness, dispersion, metallic ion passivation and a better inhibition effect. The butter inhibitor is prepared from the following raw materials of: 1 to 30 percent of antioxidant, 5 to 40 percent of polymerization retarder, 2 to 30 percent of dispersant, 5 to 25 percent of metallic ion passivating agent and the balance of solvent; and the butter inhibitor is prepared by heating, uniformly mixing and cooling the materials.