A kind of polysiloxane defoamer polysiloxane were synthesiszed by the hydrosilylation reaction with polyethylene oxide ethyl polyethylene oxide propyl ether (F - 6) and lauryl methacrylate(LMA) containing hydrogensilicone oil as material, using Karstedt the catalyst. The structure of the molecular of the synthesized defoamer were characterized by infrared (IR) spectrum and nuclear magnetic resonance (1HNMR) spectrum. The author determined the optimum reaction conditions through by investigating the effects on the reaction conversion of the molar ratio of silicon-hydrogen bonds with the carbon-carbon double bond，reaction temperature，reaction time，amount of catalyst. The best technology of the product achieved 94.78 % with the optimum reaction conditions that the molar ratio of silicon-hydrogen bonds was 1:1.20, the amount of catalyst was 50 μg/g, the reaction temperature was 90 ℃,and the reaction time was 8 h. Application test show that the defoamer defoaming and inhibiting performance superior to market similar products.