A kind of self-emulsifying defoaming agent using two polyether modified silicones T-258 and 7302 as the main active components, hydrophobic silica, sodium carboxymethyl cellulose (CMC), deionized water as the auxiliary materials, was obtained using phase inversion method through adding water phase into oil phase gradually. The defoaming agent could be used in fermentation efficiently and showed well appearance of uniform and stable white emulsion under the following conditions: 30 minutes of emulsifying time，65 ℃ of emulsifying temperature, the mass ratio of T-258 and 7302 was 1∶2 (total mass fraction was 30%)， the addition amount of thickening agent was 0.8%， combined with 1% of hydrophobic silica. Meanwhile, the defoaming agent could eliminate the foam quickly in 3-5 s and had a long foam-inhibiting period of 95 minutes in the simulated amino acid fermentation system due to its excellent spreadability. Also the defoaming agent could maintain a stable and uniform state after steam sterilization at 126 ℃ for 1 h.