The invention discloses an automatic on-line bagging method and device for a plastic bag roll. The device is composed of a bag roll body, a bag pressing mechanism, a bagging mechanism, a heat sealing mechanism and a whole support. The bag pressing mechanism is located on the left side of the bag roll body, the bagging mechanism is located on the left side of the bag pressing mechanism, and the heat sealing mechanism is located on the left side of the bagging mechanism. By means of the automatic on-line bagging method and device for the plastic bag roll, annular plastic bag packaging can be subjected to an automatic mechanization bagging operation; when the bag roll is replaced, a bag only needs to be manually led into a bag opening device, then the bag can be automatically grabbed through the next process of a machine to complete the bagging operation, and heat sealing is finally carried out through the a heat sealing mechanism; automatic bag feeding, smoothing, bag opening and heat sealing are achieved, and on-line bagging packaging is completed in the mode that heat sealing, the front process and the back process are matched. According to the automatic on-line bagging method and device for the plastic bag roll, labor is released out of the high-labor-intensity working environment, the integration level is high, the occupied space is small, labor is saved, the labor intensity is greatly reduced, the packaging efficiency is improved, and the enterprise production cost is reduced.