Synthesis of thiol silicone oil using octamethylcyclotetrasiloxane and mercaptopropyl methyl dimethoxy silane as raw materials; synthesis of organosilicon based RAFT reagents using thiol based silicone oil; The polyether modified silicone defoamer is synthesized by the reaction of silicone based RAFT reagent with methyl terminated allyl group polyethers (EO and PO) through reversible addition breaking chain transfer polymerization. The optimal process conditions were as follows: the equivalent molecular weight of silicone based RAFT reagent was 2000, m (EO)∶ m (PO)=1∶4, m (organosilicon based RAFT reagent): m (ether)=1∶1.15, the reaction temperature was 75 ℃, the reaction time was 8 h, the amount of initiator was 2% of the mass of methyl terminated allyl group polyether, and the yield was 91.8%.The structure of polyether modified silicone defoamer was characterized by FTIR, NMR and GPC, and it was used in coatings and lotion to determine the defoaming and inhibiting properties.