The invention relates to the technical field of the building material, and specifically relates to a building interface agent and a preparation method and application thereof. The invention provides abuilding interface agent prepared from the following components in parts by weight: 25-35 parts of silica sol, 10-20 parts of lithium silicate aqueous solution, 2-4 parts of a wetting agent, 4-6 parts of a surfactant, 1-2 parts of a silane coupling agent, 1-2 parts of a pH value regulator, and 40-50 parts of water. The building interface agent provided by the invention can effectively permeate 3-8mm into the aerated concrete batten surface layer, and has excellent permeation seal waterproofing; the building interface agent can produce chemical reaction with certain free components in the aerated concrete batten to produce hard and compact substance, and the surface strength, the waterproofing and the breathability of the aerated concrete batten are improved; and meanwhile, the bonding strength of a plaster layer and the aerated concrete batten is improved; the building interface agent does not contain the organic solvent, is green and environmentally friendly, and suitable for being popularized and applied in the aerated concrete batten.