The invention provides a method for preparing mesophase pitch through catalytic cracking oil slurry hydroisomerization and thermal condensation. The method comprises the steps that FCC oil slurry of naphthenic crude oil or intermediate base crude oil and distillate oil which is obtained at the temperature higher than 400 DEG C through vacuum distillation serve as the raw materials, on the condition that a hydroisomerization catalyst exists, the temperature ranges from 250 DEG C to 320 DEG C, the pressure is 5 MPa, reaction is conducted for 4-8 h, and the modified raw materials are obtained. Continuous reaction is conducted on the modified raw materials for 2-10 h at the temperature ranging from 330 DEG C to 460 DEG C and at the pressure of 12 MPa, and the mesophase pitch with the high quality is obtained. According to the method for preparing the mesophase pitch through catalytic cracking oil slurry hydroisomerization and thermal condensation, the preparation technology is simple, the production cost is low, the prepared mesophase content is high and greater than 97 percent, the softening point is low, the temperature ranges from 230 DEG C to 250 DGE C, the large-wide-area linear flow optical anisotropic structure is achieved, and the spinnability is good.