Horizontal well fracturing combines the artificial fracture and a large number of structural weak surface, such as natural fracture or bedding, forming a complex network, an important section of which is shear slip fracture. Therefore, it will be of great significance to search for shale fracability. With the plane strain and intergranular fracture condition of real shale failure taken into account, and with the focus on the microscopic morphology of shear slip fracture, a method for calculating fracture toughness of shear slip fracture under plane strain was established. Compared with the experimental results, the average error of the method was 2.93%. Based on the logging data and fracture toughness index of a horizontal shale gas well, the whole wellbore continuous profile of fracability evaluation was drawn. By comparing the production after hydraulic fracturing, the reliability of this method was verified, which provided a theoretical reference for efficient development of shale gas.