A a plant composite corrosion inhibitor for an oil field and a preparation method thereof belong to the technical field of preparation of oil field chemical agents. The plant composite corrosion inhibitor comprises a first plant ingredient, a second plant ingredient, a corrosion inhibition synergist and an organic solvent. The first plant ingredient is zeaxanthin and a derivative thereof obtained by supercritical CO2 extraction of marigold; the second plant ingredient is prepared from luffa leaves, guava leaves and eclipta according to a mass ratio of 5:8:9; the corrosion inhibition synergist is prepared by mixing potassium iodide, 8-hydroxyquinoline and sodium dodecyl benzene sulfonate according to a mass ratio of 3:4:2; and the organic solvent is ethanol with a mass concentration of 82%. The composite corrosion inhibitor has a good corrosion inhibition effect, and reduces harmful chemical ingredients in the corrosion inhibitor.