MPLS which combines the flexibility of Layer3 routing with Layer2 switching is being widely developed. Unfortunately, existing MPLS specifications can only support unicast well, but lack of supporting multicast. The main difficulty in MPLS supporting multicast is how to bind labels to multicast FEC. This paper mainly presents a new mechanism for MPLS supporting IP multicast routing protocol. In the proposed mechanism, label distribution is triggered by traffic not by control messages. This mechanism makes the network's scalability and dynamic property better than the mechanism whose label distribution is triggered by control messages. The simulation of this mechanism is for further study.