The invention discloses a composite-rubber formaldehyde-free mildew-proof medium-density fiberboard and a manufacturing process thereof. The manufacturing process of the fiberboard is characterized inthat a mixed solution of waterborne polyurethane, modified vegetable protein glue, mildew preventive and a cross-linking agent is evenly sprayed into wood fibers to form sizing fibers, the sizing fibers are laid into a board blank, pre-pressing moulding is conducted at the pressure of 60-100 N/cm<2>, heat solidification is conducted under the hot-pressing conditions that the temperature is 175-230 DEG C and the pressure is 100-350 N/cm<2>, and the composite-rubber formaldehyde-free mildew-proof medium-density fiberboard is prepared; by weight, 2-5 parts of waterborne polyurethane, 2-4 parts of modified vegetable protein glue, 0.1-0.5 part of mildew preventive and 0.2-0.6 part of cross-linking agent are added into every 100 parts of wood fibers. The wood fiberboard prepared by means of themethod does not release formaldehyde and can suppress dampness and mildew.