The invention relates to a method for preparing an anionic surfactant for oil displacement and belongs to the technical field of fine chemicals. In the method, with wide-boiling fraction serving as raw materials, the anionic surfactant for oil displacement is prepared by applying the hyper gravity and liquid phase SO3 sulfonation technology. The method includes the steps of adding and stirring a solvent and a distillate from factions with the temperature ranging from 200 to 550 DEG C in a circulation stirred tank to form a mixture, and adding a sulfonating agent into the mixture; the weight ratio of the solvent to the distillate ranges from 1 to 2 to 2 to 1, and the weight ratio of the distillate to the sulfonating agent ranges from 5 to 1 to 2 to 1; a sulfonation is carried out in a hyper gravity reactor at the temperature ranging from minus 10 to 50 DEG C for 20 to 70 minutes to obtain the intermediate product, petroleum sulfonate, which is then moved into a neutralization reactor; the neutralizing agent is dripped into the petroleum sulfonate with a mass ratio of the neutralizing agent to the petroleum sulfonate ranging from 0.9 to 1 to 1.2 to 1, at the temperature controlled between 50 and 90 DEG C; the solution is evaporated at the temperature ranging from 50 to 80 DEG C. The method has advantages of wide application range, excellent performance, cheap raw materials, short production cycle and high content of active matters.