The invention discloses a vacuum thermal expansion molding process for a composite. The vacuum thermal expansion molding process comprises the following steps: cutting out the sizes of a prepreg fabric and an expanding core material or a nylon wind pipe; laying up the prepreg fabric according to design requirements of mechanics, and coating the expanding core material or the nylon wind pipe with the prepreg fabric to form a preform; placing the preform into a cavity of a die and performing die assembly and locking; vacuumizing to enable the cavity of the die and the preform in the cavity of the die to be in a negative-pressure state; (as for the nylon wind pipe, after inflation for supercharging of the nylon wind pipe) heating and keeping the temperature of the die, and maintaining the corresponding temperature and time according to curing parameters of resin in the prepreg fabric to ensure curing of the preform; and cooling down the die, degassing to eliminate negative pressure and return to the ambient atmosphere (as for the nylon wind pipe, degassing to return the nylon wind pipe to the ambient atmosphere), opening the die and taking out the molded composite product. The vacuum thermal expansion molding process for the composite can realize continuous integral-forming production, and is low in cost and environment-friendly, and the obtained product is favorable in appearance and high in strength.