The invention discloses a preparation method of maleic anhydride grafted vegetable oil acid. The preparation method is characterized by comprising steps as follows: vegetable oil acid, maleic anhydride, a catalyst and a solvent are added into a reaction kettle and heated for an Alder-Ene reaction, unreacted raw materials and solvent are boiled off through pressure reduction after the reaction, and the maleic anhydride grafted vegetable oil acid is obtained. The preparation method is simple in process, raw materials are safe, non-toxic, easy to obtain and low in cost, the conditions are mild, and the obtained product is light in color and can be used for the next step, so that the industrialized implementation is facilitated.