The invention relates to a multifunctional additive for an oil-based drilling fluid and a preparation method of the multifunctional additive. Xylene is adopted as a reaction solvent, concentrated sulfuric acid is adopted as a catalyst, cocoyl diethanolamine is grafted onto humic acid by virtue of esterification reaction and thus the chemical modification of humic acid is achieved, as a result, humic acid molecules have good oil solubility, the modified humic acid molecules contain a large number of acylamino groups RCONR2, ester groups -COOR, -OH, -COOH, -C=O and -O- and other groups, N and O atoms in the groups can form hydrogen bonds with H atoms on other polar groups, interactions occur between the molecules by virtue of the hydrogen bonds and electrostatic attraction and thus a network structure with a large specific surface area is formed and a three-dimensional network structure is formed between the long alkyl straight chains in the molecules by virtue of an association effect. The multifunctional additive product has the advantages of filtration reducing property, increased shearing force, high-temperature resistance, calcium ion resistance and no viscosity increasing and the rheological property of the slurry can be maintained at a good state.