The invention discloses a composition used for reducing hydrocarbon oil metal content and an application thereof. The composition contains acid, a cyan-containing compound, aromatic hydrocarbon and a polyoxyethylene-polypropylene oxide segmented copolymer, the acid is selected from water-soluble phosphonic acid and water-soluble carboxylic acid, the polyoxyethylene-polypropylene oxide segmented copolymer takes phenolic resin or phenol-amine resin as a starting agent, and the cyan-containing compound is selected from aromatic nitrile with at least one hydrogen atoms substituted by C3-C8 branched-chain alkyls on aryl, and aralkyl nitrile with at least one hydrogen atom substituted by C3-C8 branched-chain alkyl on aryl. The invention also provides a method for reducing hydrocarbon oil metal content, which comprises mixing hydrocarbon oil with components in the composition and water, and a mixture is subjected to oil-water separation to obtain an oil phase and a water phase. The composition can effectively reduce the metal content of hydrocarbon oil, especially removes the calcium and iron in residuum.