A strap-feeding mechanism for an automatic strapping machine including a quick strap-feeding mechanism for quickly feeding a strap into a strap rail on the automatic strapping machine and an automatic strap-refeeding mechanism for clamping a strap which slides off the strap rail during a pull-back operation, so that the strap is prevented from further sliding backward and is automatically refed into the strap rail again. The quick strap-feeding mechanism mainly includes a stationary panel and a cover panel. The stationary panel is provided with a strap guiding channel which has a lower end connected to a strap outlet of a strap-storing power unit, an upper end connected to a strap inlet of a strap-feeding unit, and a straight middle section having only one side wall. The cover panel is fixed to a front of the stationary panel and has a movable leaf which can be turned to extend into the stationary panel and forms another side wall of the straight middle section of the strap guiding channel. The strap refeeding mechanism includes a driving wheel and an opposite rotational wheel extending into the upper end of the strap guiding channel to together clamp the strap slidden off the strap rail and then refeed the strap into the strap rail.