The invention discloses a preparation method of a glass fiber modified impregnating compound, and belongs to the technical field of fiber-reinforced composite materials. The preparation method comprises the following steps: performing graft modification by taking glass fiber as a silane coupling agent (KH550) in an impregnating compound; performing graft modification on silicon dioxide; preparingan impregnating compound emulsion. The preparation method is characterized in that the silane coupling agent in the impregnating compound is subjected to graft modification, and modified silicon dioxide is added, so that the prepared impregnating compound emulsion is uniform in particle size distribution which is less than 500 nm, and is smaller in surface tension which is about 16 mN/m. Moreover,the glass fiber coated with the impregnating compound is improved in mechanical performance, and the interface bonding firmness between the glass fiber and resin is enhanced. The method is simple inprocess, and can be conveniently applied to glass fiber-reinforced composite materials.