The invention relates to a manufacture method of a box-shaped structural main beam made from a composite material. The method comprises the following steps of: A, manufacturing a male mold, a female mold and an assistant tooling balance press plate; B, paving and gluing a prepreg of the composite material on the male mold, and vacuumizing and previously compacting under the normal temperature; C, laying the previously-compacted composite material on the female mold; D, combining the assistant tooling balance press plate, the male mold and the female mold from top to bottom, and packaging in a vacuum bag; E, putting the vacuum bag into a hot-press tank for vacuumizing and checking the air impermeability; and F, rising and keeping the temperature of the hot-press tank, keeping the pressure and rising the temperature after the tank is pressured, keeping the pressure and the temperature, cooling, releasing the pressure, and removing the male mold, the female mold, the assistant tooling balance press plate and a substrate to finally obtain the box-shaped structural main beam made from the composite material. According to the manufacture method, each molded surface region of the prepreg of the composite material can be pressured within a resin gel time, and the loosing of parts caused by the small pressure can be avoided.