The invention discloses an active admixture, a preparation method and an application for the active admixture, and concrete, belonging to the technical field of building materials. The active admixture comprises the following components in parts by weight: 10-30 parts of Bayer process red mud, 5-15 parts of quicklime powder and 80-120 parts of limestone powder, wherein the water content of the Bayer process red mud is 15%-35%, and the mass content of aluminum oxide in the Bayer process red mud is 20%-40%. According to the invention, the Bayer process red mud and the limestone powder are used as main raw materials; the problem of difficult dehydration for the Bayer process red mud is solved by utilizing the principle of a violent reaction of quick lime and water; hydrated sodium aluminosilicate with high content and relatively good water solubility in the Bayer process red mud is utilized to react with the limestone powder to produce hydrated aluminosilicate; thus, the hydration activity of the active admixture can be improved; and the active admixture can completely replace fly ash to reduce the production cost of the building materials like the concrete.