The invention discloses an automatic screw tightening mechanism for frame assembling, and belongs to the field of frame assembling and processing mechanical equipment. A screw guiding mechanism, a screw flat pushing mechanism and a screw transferring mechanism of the automatic screw tightening mechanism are sequentially, horizontally and fixedly arranged on one side of a screw transfer bracket inthe horizontal direction, a screw circulation mechanism is vertically arranged on one side of the screw guiding mechanism, a frame conveying through hole of a square structure is vertically arranged on the position, on one side of the screw transferring mechanism, on the screw transfer bracket, a screw tightening mechanism is vertically arranged on the portion, on one side of the upper end of theframe conveying through hole, of the screw transfer bracket, the two ends of a screw transfer belt are wound around yarn guide rotary rollers connected to the two sides of the screw transfer bracket,and the upper side of the screw transfer belt is provided with a screw hopper of a flat structure. The automatic screw tightening mechanism is reasonable in structure design, screws can be arranged and loaded smoothly and orderly, the screws can be accurately transferred to desired positions one by one, the screws can be screwed and machined efficiently, frame continuous and automatic assembling is achieved, and the production and assembly needs are met.