In the field of communications, a method and a system for packet transmission in an optical communication system, and an Optical Line Terminal (OLT) are provided. The method includes: receiving, from a port, a first uplink packet from an Optical Network Unit (ONU), where the first uplink packet includes a Logical Link Identifier (ID); determining a first ID corresponding to the first uplink packet, where the first ID includes an ONU ID or a Port ID; acquiring a Multiprotocol Label Switching (MPLS) tunnel label according to the Logical Link ID and the first ID; and transmitting a second uplink packet on an MPLS tunnel corresponding to the MPLS tunnel label, where the second uplink packet includes data in the first uplink packet and the MPLS tunnel label. The system includes an ONU and an OLT. By establishing a corresponding relation of a logical link, created between the OLT and the ONU, and the MPLS tunnel, mapping between the logical link and the MPLS tunnel is realized, so that the service configuration has an unlimited capacity and becomes more flexible.