The invention relates to a pretreatment method for hydrocracking raw material to improve the processing property of coker gas oil to be equivalent with straight-run refining wax oil through remove polar impurities, such as coke powder, colloid, heterocyclic benzenoid, and basic nitrogen compound, from coker gas oil, which includes the following steps: the pretreatment agent and the coker gas oil are mixed under 20-120 DEG C according to 0.1-10 percent by weight proportion; the pretreatment agent includes 49-89 percent of active components, such as hydrochloric acid, nitric acid, perchloric acid, sulfuric acid, phosphorous acid or phosphorous anhydride; 10-50 percent of cosolvents, such as ferric chloride, glycerol, ethylene glycol or DMF; and 0.1-10 percent one or more of polyacrylamide, polyaluminium chloride, polymeric aluminum ferric chloride, sodium meta-aluminate, sodium sulfate, sulfuric acidaluminum, aluminum potassium sulfate, and sodium silicate; the mixed materials are gravity settled in a settling tank, the sedimentation temperature is 60-150 DEG C, and the time is 30-120min, refined coker gas oil can be directly blended into hydrocracking device to decrease the initial temperature of hydrocracking prerefining section by more than 10 DEG C or improve the device processing capacity by more than 30 percent.