The sealing device involves an elongated release base member (1) having a separation line (13) along the release base member (1)'s length. A double-sided adhesive layer (2) is attached to a side of the release base member (1) so that the release base member (1) may adhere to an inner side of a plastic courier bag (4)'s sealing flap (40) and seal the plastic courier bag (4). A reinforcing tape (3) may be further applied to an outer side of the plastic courier bag (4)'s sealing flap (40). The plastic courier bag (4) may be quickly and conveniently opened by tearing down a pull strip (12) of the sealing device. The plastic courier bag (4) may be reused multiple times by applying new, unused sealing devices.