The invention belongs to the technical field of environment remediation and particularly discloses a compound remediation agent for cleaning petroleum-contaminated soil. The compound remediation agent is prepared from plant enzymes and biochar, wherein the biochar is formed by calcination of kelp in the oxygen isolation environment; and the biochar is added into the plant enzymes fermenting for 2-3 days, fermenting is continue to conducted, filtering is conducted, then obtained mixed liquid is placed in the ultraviolet environment to be sterilized, and thus the compound remediation agent is obtained. An application method of the compound remediation agent comprises the steps that water is added into the compound remediation agent to be diluted by 3-5 times, and then the diluted compound remediation agent is sprayed on the surface of the soil; the soil is turned over and irrigated; and a diluted solution of the compound remediation agent is sprayed once every 7 days, and the spraying density is 0.3-0.5 kg/m<2>. Compared with an existing biological or chemical or physical remediation method, when the petroleum-contaminated soil is remedied by the compound remediation agent, the characteristics that the quick effect is achieved, the technical requirement degree is low, and secondary pollution is avoided; and the compound remediation agent has the advantages of being easy to prepare, low in cost, environmentally friendly and rich in source.