The invention relates to an organosilicone antifoaming agent and especially relates to an acid and alkali-resistant organosilicone composite antifoaming agent and a preparation method thereof. The acid and alkali-resistant organosilicone composite antifoaming agent comprises, by weight, 9.0-40.0 parts of polysiloxanes, 0.3-1.5 parts of nanometer silica, 0.5-2.0 parts of a thickening agent, 0-7.0 parts of a composite emulsifier, 0.5-2.5 parts of low molecular alcohol, 0.05-0.40 parts of a silane coupling agent, 0.02-0.10 parts of a basic catalyst, 0-0.15 parts of an acidic terminator, 50-90 parts of water and 0-1.5 parts of a medium material. The polysiloxanes comprise one or more of an inert organopolysiloxane mixture A, an active organopolysiloxane mixture B, a modified polysiloxane mixture C and a high-functionality polysiloxane mixture D. The acid and alkali-resistant organosilicone composite antifoaming agent is widely used in chemical fiber and papermaking industries, has a very high defoaming capability and has stable chemical properties in an acid-base system.