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The invention relates to a solvent-free method for preparing a polyether modified polysiloxane antifoaming agent. The solvent-free method is characterized by comprising the following steps: 1, adding siloxane-oxyalkylene copolymer which is spirally evaporated to remove water to a reaction kettle, then filling nitrogen, and adding an antioxidant BHT (Butylated Hydroxytoluene); 2, adding a platinum catalyst to the reaction kettle, and heating the reaction kettle and stirring, wherein the provided temperature for heating is at 50 to 60 DEG C; 3, adding low-hydro silicone oil and a 4A molecular sieve to the reaction kettle for starting hydrosilylation reaction, and controlling the temperature in the reaction kettle to reach 70 to 110 DEG C, then stirring at constant temperature for 1 to 2 hours; 4, adding hydrochloric acid or sulfuric acid to the reaction system, then stirring to neutralize, and reacting for 1 to 1.5 hours at 85 to 95 DEG C; and 5, spirally evaporating to obtain the reacting product which is the polyether modified polysiloxane after the reaction. The solvent-free method is high in chemical conversion rate; and the prepared polyether modified polysiloxane antifoaming agent is transparent and high in purity, so that the antifoaming performance of the polyether modified polysiloxane antifoaming agent is high.