The invention discloses a method for screwing down an oil injector assembly. The method for screwing down the oil injector assembly comprises the following steps of (1) carrying out accurate grinding on a sealing face of an oil injector body, cleaning the oil injector body and a fastening nut, carrying out a preparatory process before screwing-down operation, evenly stirring an antioxidant and deionized water, adding the mixed liquid of the antioxidant and the deionized water to a liquid storage box of a waxing machine, pumping the mixed liquid into a waxing barrel of the waxing machine, immersing the fastening nut in the mixed liquid, discharging the mixed liquid, rotating the waxing barrel in a high-speed mode to throw liquid so as to dry the fastening nut, preassembling the oil ejector assembly, (2) setting a screwing-down program of an automatic screwing-down machine to be an angle control program, setting a screwing-down angle in the angle control program to be 32 degrees, setting initial screwing-down torque to be 10N*m, (3) carrying out high-speed grinding-in on the oil ejector assembly, and (4) carrying out performance tests on the oil ejector assembly. According to the method for screwing down the oil injector assembly, axial pressure scatter difference produced in the process of screwing down the fastening nut is about one third of that in the prior art. When the method for screwing down the oil injector assembly is adopted, under the action of the same screwing-down torque, axial pressure produced in the process of screwing down the fastening nut is increased by 30%-50%, and sealing performance of the oil ejector assembly is improved.