This invention discloses a method for preparing environmentally friendly lightweight fiber wallboard. The raw materials comprise: diffusant 3%, ferrous sulfate 1%, magnesium oxide 20%, magnesium chloride 15%, industrial waste (industrial slag or fly ashes) 31%, and grass powder (pulverized haulm, wheat straw, bamboo leaves or crop straw) 30%. The method comprises: pulping the raw materials, loading into a mold, pressing, flattening, molding, flattening, polishing, demolding, repairing, and examining. The lightweight fiber wallboard has such advantages as high resistance to halogenation and frost, high strength, high anti-shock performance, high sound insulation performance, and high stability.