The invention belongs to the technical field of oil-based drilling treatment, and particularly relates to an extracting solvent for treating oil-based drillings and a preparation method thereof. The extracting solvent is prepared from alkyl chloride, magnesium powder and vinyl chloride in a solvent. The preparation method comprises the following steps: adding the solvent in a reaction vessel, pumping nitrogen for protection, and then sequentially adding the solvent, the alkyl chloride and the magnesium powder for reaction; then adding a catalyst and the vinyl chloride in the reaction vessel for reaction, and heating, distilling and filtering after the reaction is completed, thus obtaining the extracting solvent. According to the extracting solvent disclosed by the invention, the problems of lower circulation utilization rate, higher economic cost, serious environment pollution and serious potential safety hazard caused by volatility and low flash point of an existing conventional solvent are solved, and the extracting solvent has the advantages of high flash point, high security, good extracting effect, high recovery rate, small emission pollution, simple process technology, easiness in implementation and the like.