Abstract: Multicast routing tree, built directly upon the Backbone, can not satisfy the dynamic and symmetric requirements of multi point transfer on a video conferencing system. Thus, dynamic virtual network （DVN） is put forward as a multi point transfer platform of a video conferencing system. DVN is a virtual network connecting all participants and built dynamically on the Backbone, as it is built when multi party session like video conferencing starts and dismantled when the multi party session finishes. Besides all participant nodes, DVN also contains certainly several routing nodes that take charge of duplicating and merging streams. DVNs network like structure can meet quite well the need of dynamic and symmetric multi point transfer, and can unify dense mode and sparse mode of multi point transfer. In addition, DVN may be combined easily with multimedia transfer protocol and real time collaborative model. On the other hand, the paper presents a simple algorithm for building DVN, and explains the processing upon receiving the requests of the users dynamic join and leave. Here also narrated are DVNs routing structure, how multi point transfer is realized upon DVN, and how DVN supports video conferencing. Finally, comparison with the existing method, which builds directly multicast routing tree upon the Backbone, such as DVMRP, CBT and PIM, and a performance analysis are made for DVN.