The invention discloses a synthesizing method of gamma-aminopropyl triethoxysilane, comprising the following steps of: (1) adding a toluene solution dissolved with a catalyst and the gamma-aminopropyl triethoxysilane to a reaction still under the condition of vacuum, adding ammonia in a stirring state, and increasing the temperature of materials inside the reaction still to 60-70 DEG C to react for 4-7 hours at the reaction pressure of 0.5-3 MPa; (2) after amination reaction is finished, filtering, separating and removing side products, and adding filter liquor to a distilling still; (3) after the top temperature of the distilling still is more than or equal to 102 DEG C, reducing the temperature of the materials inside the distilling still to be less than or equal to 80 DEG C, and carrying out reduced pressure distillation at vacuum degree of more than or equal to 0.099 MPa so as to obtain the gamma-aminopropyl triethoxysilane after the reduced pressure distillation. The invention enhances the purity of synthesized products by more than 99 percent (GC) and expands the application fields to industries, i.e. electronic and electric products, high-performance catalyst preparation, nanometer materials, superfine dispersing materials, and the like which have higher requirements for product purity.