The invention provides a method for preparing insulation bricks by using ramie fiber and riverway sludge, and relates to the field of building materials. The method comprises the following steps: preparing the ramie fiber; heating trichloromethane to 75-85 DEG C, adding PVC while stirring, continuing stirring until the PVC is completely dissolved, cooling to room temperature, then immersing the ramie fiber for 15-20min, then taking out, and drying to obtain PVC-coated ramie fiber; excavating the riverway sludge, then flushing by using river water till a flow state, then conveying into a sludgetank by using a pressure pump, adding carbon powder, leaving to stand, then extracting a supernatant, excavating soil in the bottom layer, drying, then crushing, adding into a mixing tank, adding tapwater, fly ash, the PVC-coated ramie fiber, a polycarboxylate water reducer and Portland cement, heating up to 40-60 DEG C and uniformly stirring; sending a mixed material into a brick press, pressing into brick blanks, performing steam curing on the brick blanks in an autoclave for 5-6 hours, then drying till the moisture content is lower than or equal to 12%, and curing for 15-30 days. The prepared insulation bricks have the characteristics of high strength, no cracking, good insulation and the like and have extremely high economic and social benefits.