Abstract：A new type of (AB)n polyether-block-polysiloxane (PBMS) was synthesized by hydrosilylation reaction between Si-H terminated polydimethylsiloxane and allyl- polyoxyethylene-polyoxypropylene (AAF) in the presence of H2PtCl6-isopropyl alcohol. And its chemical structure was characterized by FT-IR spectroscopy. The defoaming agent emulsion was formulated from polydimethylsiloxane, hydrophobic silicon dioxide, MQ silicone resin, the PBMS and so on. Starting from preparation of silicone defoaming surfactant, conditions that affect the defoaming and inhibiting performance, such as mass fraction of these defoaming surfactants, PBSO with different structures, the length of silicon chain and molecular weight of AAF polyether, were investigated. Experiment results showed that the defoaming agent can rapidly broke down 100mL bubble in 7.8″, with bubble-restrain time of 45.8′, and no delamination, no emulsion breaking under the base condition(pH =12) and at high temperature(65°C) for 24h.