A tightening auxiliary device for a strapping belt includes a main body which is mounted to a strapping machine. One end of the main body is a stop portion provided with a strapping buckle. When the strapping machine is used to strap a box, the strapping belt is wound around the box, and a first end of the strapping belt is fixed to a first side of the strapping buckle, and a second end of the strapping belt is wound around a second side of the strapping buckle and fitted on a rotary shaft of the strapping machine. During the tightening operation, the stop portion holds against the strapping belt and the first side of the strapping buckle, so that the strapping machine can pull the second end of the strapping belt effectively for tightening the strapping belt to be closely attached to the box, enhancing the strapping effect.