The invention relates to a preparation method of ethyl trimethoxy silane, aiming at solving the technical problem that a process and a device can be simple, usage and cost of a catalyst can be reduced, synthetic yield can be improved and desired products can be easy to be separated without adding additional solvent. The method in the invention comprises the following steps: 1) adding trimethoxy silane, catalyst and cocatalyst to a pressure-proof reaction kettle, wherein the mass ratio of the usage of catalyst to trimethoxy silane is 1*10<-6>-0.10, and the mass ratio of the usage of cocatalyst to trimethoxy silane is 5*10<-6>-0.10; 2) heating up to 0-150 DEG C under stirring, then continuously introducing ethylene for reaction; 3) stopping the reaction when the pressure in the reaction kettle is not decreased to obtain the synthetic product of ethyl trimethoxy silane; and 4) carrying out separation and extraction on the synthetic product to obtain ethyl trimethoxy silane.