The present invention relates to a joint adapter for a power drill screw driver having a driving shaft, a slide socket, a compression spring, a packing ring and a screw driver bit. The driving shaft is provided with an insertion groove and an insertion piece. A neck is located in the middle of the driving shaft. A strong magnet is fixed at the front end of the axial insertion hole. A clamping ring is arranged at a proper place of the axial insertion hole. The compression spring is inserted around the neck of the driving shaft and rests on the clamping ring of the slide socket. The packing ring closely sits at the end of the neck of the driving shaft in a tight state while the compression spring presses against the packing ring so that the compression spring and the slide socket are located in place. A plurality of screw driver bits in a plurality of shapes and sizes each being selectively inserted into the insertion groove of the driving shaft for tightening the screws.