The invention discloses a preparation method of biomass solid fuel. The method includes the steps of adding cassava lees into a smashing machine to be smashed to be 40-60 mesh to obtain cassava lee powder, adding the cassava lee powder, magnesium oxide, acetylene sludge, attapulgite and pucherite into a stirring device, adding a proper amount of water to be stirred into paste, putting the paste into a muffle furnace to be sintered for 3-5 h at a temperature of 500-600 DEG C to obtain carbide, evenly mixing the carbide with polyurethane rubber and polyurethane asphalt, conveying the obtained mixed materials to a forming machine to be press-formed at a temperature of 120-140 DEG C and pressure of 60-80 MPa to obtain formed fuel, drying the formed fuel, cooling the fuel to the room temperature, and packaging the fuel to obtain the biomass solid fuel. The method has the advantages of being simple in preparation process, low in energy consumption, high in production efficiency, high in forming rate, low in cost and the like; the prepared biomass solid fuel is low in density, high in heat value, small in ash content, small in production internal gap, free of loosening, good in humidity resistance, capable of being stored for a long time and suitable for industrial production.