The invention discloses a method for preparing a large-size sheet by using small-diameter wood. The method comprises the steps of: cutting small-diameter wood with a diameter of 6-11cm into wood sections with lengths of 1m, grouping into 5 groups according to the diameter of the small head and the 10mm difference, and drying until the water content is 12 percent; sawing the wood sections into rough regular hexagonal prisms, inscribed circles of bottom surfaces of which are 46mm, 56mm, 66mm, 76mm and 86mm, extending through a finger joint tenon and cutting according to the specification, milling into refine regular hexagonal prisms, inscribed circles of bottom surfaces of which are 40mm, 50mm, 60mm, 70mm and 80mm; symmetrically splitting the regular hexagonal prisms into quadraprisms through the maximum diagonal plane; symmetrically splitting the regular hexagonal prisms into pentagonal prisms through a plane vertical to the side surfaces of the hexagonal prisms; and selecting the quantities of the refine regular hexagonal prisms, and the pentagonal prisms and the quadraprisms, thereby obtaining a rectangular large-section glued wood beam through coating, assembling and bidirectional-pressurizing, and sawing into the large-size sheet with the thickness of 5-20mm. According to the method, the utilization rate and the use value of the small-diameter wood are improved.