The invention provides a multifunctional metal surface composite treatment agent and a preparation method and a use method thereof. The multifunctional metal surface composite treatment agent comprises the following ingredients: water, manganese phosphate, magnesium chloride, sodium chloride, sodium alkyl benzene sulfonate, sodium tripolyphosphate, vegetable tannin, ethylene glycol, hydroxy acid, tartaric acid, methyl acid, polybrominated biphenyls, polybrominated diphenyl ethers, veratrole, sodium phosphate and peregal. Traditional metal surface treatment needs to adopt such processes as deoiling, dedusting and passivating, however, by use of the multifunctional metal surface composite treatment agent, only one process is needed, that is, effects of the multiple processes of the traditional metal surface treatment method can be achieved so long as metal to be treated is immersed into a soaking pool in which the multifunctional metal surface composite treatment agent is placed to be soaked for a period of time, so that the manual cost and the equipment cost are greatly lowered, and the working efficiency is improved.