A U-type bicycle handbrake assembly structure is provided so that the brake arms could be quickly and easily positioned. The invention cuts a planar surface on the fitting tube welded to the bicycle frame. The planar surface is parallel to the brake pad's contact surface to the wheel rim. The invention then configures a protruding plate on the adjusting seat along the adjusting seat's axial direction. When a brake arm is pin-joined to the fitting tube of the bicycle frame, the brake arm could be positioned quickly and easily by arranging the axial protruding plate of the adjusting seat to match against the planar surface of the fitting tube.
