In order to improve the scalability of multicast protocol in large-scale distributed interactive systems, a content-based bi-directional shared multicast routing protocol is presented, which is called CBSMRP (content-based bi-directional shared multicast routing protocol). Combined with active routing methods and content-based publish/subscribe pattern, this new protocol supports active routing and bi-directional filtering according to the content of packages in a bi-directional shared multicast tree based on CBT (core-based tree) structure, which cannot only solve the problems in the allocation and maintenance of multicast addresses, but also efficiently reduce the network load. Experimental results and application show that the protocol is scalable enough to meet network communication requirements of large-scale distributed interactive systems.