The invention provides a silicon-removing agent, a preparation method and an application thereof, belonging to the technical field of silicon-containing sewage treatment. The silicon removing agent includes the following components by weight: 73-77 parts of polyaluminum chloride, 10-14 parts of sodium metaaluminate, 4-8 parts of polyaluminum sulfate, 3-7 parts of polyferric sulfate, activated alumina 1‑3 servings. After the silicon-containing sewage is treated with the silicon-removing agent of the invention, the soluble silicon in the sewage is reduced to a certain extent, the recycling of sewage is not affected, nearly zero discharge is achieved, and multiple functions such as silicon-removing, coagulation, and turbidity removal are achieved. The silicon removing agent of the invention has high purity, less impurities, less dosage, low sludge amount, high silicon removing efficiency, is not affected by changes in sewage water quality, has high running stability, low running cost, and is safe and convenient to operate.