Abstract: A new kind of network layer packet forwarding technology, the multiprotocol label switching（MPLS）, is discussed in the paper. MPLS combines network layer packet forwarding with link layer frame switching for L3 packet relaying. This technology not only greatly improves the packet forwarding capability in intermediate systems, but also enhances the network layer routing system’s scalability. MPLS lacks formal description at present and this makes it difficult to carry out MPLS’s problem analysis. Following the analysis of MPLS system principles and its core mechanisms, a formal description of the MPLS system model using algorithm language is put forward. The work done in the paper will be helpful to further analysis and problem solving for MPLS.