Abstract: IP multicast confronts a severe scalability problem when there are large numbers of active multicast groups in the network due to state explosion and control explosion. In backbone networks, this state scalability problem is exacerbated, since there are potentially enormous multicast groups crossing backbone domains. To improve the state scalability of multicast in backbone domains, a novel scheme, called AMBTS（aggregated multicast based-on tree splitting） is proposed, which uses the concept of aggregated multicast, and employs spanning tree splitting before aggregating. Through analyses and simulations, it is shown that AMBTS can greatly improve state scalability: the number of aggregated trees is bounded in a small fixed number, and the multicast routing entries in transit nodes can be dramatically decreased.