The invention discloses a lightweight brick produced by using papermaking sludge as a raw material and a preparation method thereof, and belongs to the fields of building materials and environmental friendliness. The papermaking sludge is used as the raw materials in the lightweight brick. The method comprises the following steps of: sequentially adding a base material, a cross linker and a foaming agent into a stirrer in a ratio, fully stirring the materials, mixing the materials uniformly, feeding the mixture into a brick making machine for molding, curing for 4 to 6 days in a curing room at normal temperature, and performing finishing treatment on the cured lightweight brick to obtain a finished product. The volume density of the prepared lightweight brick can reach 400 to 700Kg/m<3>; because the lightweight brick is formed by cross linking and curing of cellulose, the lightweight brick has high strength and elasticity, and the seismic resistance and the bending resistance of the lightweight brick are further superior to those of a concrete lightweight brick; and the heat preservation performance of the lightweight brick is also superior to that of the concrete lightweight brick.