Software Defined Network (SDN) provides a new solution for network virtualization, which can virtualize a set of infrastructures to multiple logical networks to meet different network requirements. This paper studies the mapping of virtual network rules to physical network rules when multiple physical switches are virtualized into one big switch in SDN network virtualization. Considering link load, rule distribution and node load, this paper proposes a three-stage rule mapping optimization algorithm. First, according to the rules of the virtual network, the multicast source node and a destination node set are located, and a rule mapping tree is generated by using the MPH algorithm. Then, the rules of the virtual network rules are deployed to multiple nodes of the physical network. Finally, with the consideration of the node load, the rule deployment is finely-tuned to generate the virtual rule mapping strategy. Simulation results show that compared with direct egress node deployment the average number of network node rules is reduced by more than 40%.