The invention discloses a method for synthesizing high-molecular-weight ethyl polysiloxane. The method comprises the following steps: adding reaction monomers, catalysts, promoter and modifier by a certain ratio to a reaction bottle; increasing the temperature to 70 to 150 DEG C at the reduced pressure or constant pressure, and reacting for 4h to 8h; and removing low-boiling-point substances by heating under the vacuum condition, to obtain ethyl polysiloxane. The method of the invention has the advantages of simple reaction conditions and solvent-free reaction; the monomers, catalysts and promoter are added to the reactor at one time in the reaction, so that the reaction can be conducted at a lower temperature within shorter reaction time to obtain the high-molecular-weight ethyl polysiloxane; and the method of the invention further has the advantages of high yield, controllable molecular weight and simple post-treatment.