The invention discloses a preparation method and an application for a lightweight high-strength and heat-preserving wall material with coal gangue as a main material. The preparation method comprisesthe following steps: 1) high-energy ball milling: a step of subjecting the coal gangue to ball milling so as to obtain a powder, wherein the particle size of the powder is required to be smaller thanthe size of the mesh of a 150-mesh standard sieve; 2) batching: a step of adding calcium raw material powder, coal gangue powder, water with a required amount and a sodium hydroxide reagent in a specific proportion into a closed reaction vessel; 3) hydrothermal synthesis reaction: a step of carrying out a hydrothermal synthesis reaction according to preset reaction temperature, reaction time and stirring speed so as to obtain a calcium silicate nanofiber composite powder; 4) molding: a step of drying the powder material obtained in the step 3), and carrying out dry-press molding so as to obtain a green body; and 5) low-temperature sintering: a step of subjecting the green body obtained in the step 4) to heating, heat-preserving and cooling in a high-temperature device so as to obtain a final product. The wall material prepared by using the method provided by the invention has the characteristics of light weight and high strength, has heat-preserving and sound-insulating performances atthe same time, and can be used as an interior or exterior wall building material.