The invention discloses a preparation method of a wetting agent used for an oil-based drilling fluid. The method comprises the following steps: mixing and stirring vegetable oil fatty acid and maleic anhydride or fumaric acid or acrylic acid, heating the obtained mixture to 200-220 DEG C, carrying out a reaction for 2-4 h, and cooling the obtained reaction product to 130 DEG C; adding diethylenetriamine, and carrying out heating dehydration until no water is separated out at 260 DEG C; adding the vegetable oil fatty acid with the amount being 1.1-1.25 times the first-time addition amount of the vegetable oil fatty acid, heating the obtained mixture to 220 DEG C, and separating out water until no water can be separated out; and cooling the obtained material to 130 DEG C, adding xylene with the amount being 0.7 times the weight of the first-time addition amount of the vegetable oil fatty acid in order to realize dilution, and carrying out uniform stirring. The high-temperature and high-pressure filter loss of the oil-based drilling fluid using the wetting agent after 260 DEG C high temperature hot rolling for 16 h is 2.2-4.0 ml, the change of the apparent viscosity and the plastic viscosity before and after the hot rolling is small, the oil-based drilling fluid using the wetting agent has good rheological property and low plastic viscosity, facilitates the improvement of the mechanical drilling speed, and after the hot rolling, the emulsification stability is good, and the demulsification voltage is 1000 V or above.