The invention discloses a shield mud defoaming agent. The defoaming agent comprises the following components in parts by weight: 20-32 parts of phenyl silicone oil, 8-15 parts of diethylenetriaminepenta(methylene-phosphonic acid), 1.5-4 parts of palm oil, 3-6 parts of fumed silica, 2.5-8 parts of a composite emulsifier, and 0.5-3 parts of a stabilizer. The shield mud defoaming agent is obtained bythe coordination of phenyl silicone oil, diethylenetriaminepenta(methylene-phosphonic acid), palm oil, fumed silica, the composite emulsifier and the stabilizer according to a certain ratio, and hasgood defoaming and anti-foaming performance, and the resource utilization of the shield mud is relatively well carried out. The invention also discloses a preparation method and a using method of theshield mud defoaming agent.