The invention discloses a bag molding mechanism for a fully automatic vacuum packaging machine. The vacuum packaging machine comprises a vacuum reciprocating mechanism, wherein the vacuum reciprocating mechanism comprises a main box which can move leftwards and rightwards and side boxes which are positioned and fixed on the left side and the right side of the main box; the leftward and rightward movement of the main box can be simultaneously combined with one side box to form a vacuum chamber and is an open packaging station with the other side box; two bag molding mechanisms exist; and each bag molding mechanism comprises a front supporting plate, a rear supporting plate, a front air bag pressing plate, a rear air bag pressing plate, an inner pressing plate, an outer pressing plate, a front small air bag and a rear small air bag; the front supporting plate and the rear supporting plate are fixed at the front end and the rear end of a side face of the main box respectively; the front air bag pressing plate and the rear air bag pressing plate are fixed on the front supporting plate and the rear supporting plate respectively; the two ends of the inner pressing plate and the outer pressing plate are movably connected with the front air bag pressing plate and the rear air bag pressing plate, so that the front air bag pressing plate and the rear air bag pressing plate are parallel to the side face of the main box; and the front small air bag and the rear small air bag are sandwiched between the front air bag pressing plate and the rear air bag pressing plate and between the inner pressing plate and the outer pressing plate respectively.