A red mud-based sewage treatment agent and a preparation method thereof, a red mud-based ceramsite concrete and a preparation method thereof, and applications, the agent including the following components: 80-90% of a solid waste complex after acidification treatment, 2-15% of an alkali activator, 0-1.0% of a water reducer, 0.5-2.0% of an ultrafine additive, and 1.0-5.0% of a treatment agent. The solid waste complex includes: at least one of red mud powder, blast furnace slag powder, fly ash, coal gangue powder and iron tailings powder. First, the red mud is modified to prepare an intermediate product, namely the red mud-based sewage treatment agent, which is used for absorption of pollutants in sewage and then used as a raw material again to prepare a final product, namely the red mud-based ceramsite concrete. Progressive comprehensive utilization of the red mud is realized, and adsorbed pollution factors can be sealed in the final product.