The invention provides a preparation method of petroleum base mesophase pitch with even high mesophase content. The FCC oil slurry of naphthenic crude oil or intermediate base crude oil serves as the raw material, under the action of a catalyst, haloalkane is mixed with the raw material, the temperature is increased to range from 280 DEG C to 350 DEG C, the pressure ranges from 0 MPa to 2 MPa, reaction is conducted for 5-20 h, and modified oil is obtained. Distillate oil, obtained at the temperature higher than 400 DEG C, of the modified oil is taken, the distillate oil is mixed with a co-carbonization agent, the temperature ranges from 360 DEG C to 480 DEG C, the reaction pressure ranges from 4 MPa to 10 MPa, reaction is conducted for 2-10 h, and the high-quality mesophase pitch is obtained. According to the preparation method of the petroleum base mesophase pitch with the even high mesophase content, preparation technology is simple and easy to operate, the anisotropic structure of the prepared mesophase pitch is uniform, the mesophase content is high and larger than 96 percent, the softening point is low, the temperature ranges from 240 DEG C to 260 DGE C, and the spinnability is good.