The invention provides a method for manufacturing a soybean protein modified adhesive formaldehyde-free medium-density fiberboard, and belongs to the technical field of density fiberboard manufacture. The method comprises the following steps: heating, stirring and mixing raw materials (soybean pulp, a denaturing agent, a crosslinking agent, a defoaming agent, a preservative and water) to react; preparing the soybean protein formaldehyde-free adhesive by virtue of a colloid mill and a homogenizer; adding a water-proofing agent, a curing agent and a buffering agent into the soybean protein formaldehyde-free adhesive, stirring and mixing at room temperature, and uniformly mixing with wood fibers; performing drying, paving and hot-pressing to prepare the soybean protein modified adhesive formaldehyde-free medium-density fiberboard. The method is environment-friendly and non-pollution; the soybean protein modified adhesive formaldehyde-free medium-density fiberboard is high in adhesion strength, good in water proofing, low in price, low in viscosity and easy to treat and process, can be subjected to hot-pressing and cold-pressing in application, and is not easy to crack. The soybean protein modified adhesive formaldehyde-free medium-density fiberboard can be adhered to fibers and the like, and the processing process can be controlled, and an environment-friendly fiberboard can be manufactured.