The invention discloses a biomass fuel based on sludge, straw and raw coal, a preparation method of the biomass fuel and application of a fuel slag, and belongs to the field of resource utilization of solid waste. The biomass fuel is formed by mixing, pressing and molding the sludge, the straw and the raw coal; and the weight ratio of the sludge, the straw and the raw coal is (1-3): (1-3): 3. The preparation method comprises the following steps of: adding limestone to the dehydrated sludge subjected to application of an aluminum coagulant, wherein the mass percentage of the limestone in the sludge is 1 to 3 percent; mixing the sludge and the straw to aerobically compost; mixing the raw coal with the mixture of sludge and straw, pressing and molding to obtain the biomass fuel. After the biomass fuel disclosed by the invention is burnt, the maximum absorption capacity of the slag exceeds 25g/Kg and can be used as substitutes of filler materials for artificial wetlands. According to the biomass fuel based on sludge, straw and raw coal, the preparation method of the biomass fuel and the application of the fuel slag, water in the raw material can be hydrated quickly, heavy metals can be passivated, the emission of SO2 in combustion of the raw coal can be reduced, and the burnt slag of the biomass fuel has high contents of calcium, aluminum and Ferrum with strong absorption capacity to phosphor and can be used as a phosphorus removing agent during sewage treatment.