A method and corresponding device for determining forwarding rule for data packet in Virtual Private LAN Service with Provider Backbone Bridge (PBB-VPLS) network are disclosed. In the method, a value in a backbone service instance identifier (I-SID) field of the received data packet is firstly examined, then a virtual split horizon group corresponding to the data packets is determined based on the I-SID value, wherein the virtual split horizon group defines a forwarding rule for the data packets between different pseudo wire ports of the PBB-VPLS network. With the dynamic split horizon group, the method dynamically adapts to different forwarding rules for multiple I-VPLS instances with different tree topologies, and is capable of supporting multiple I-VPLS instances with different root sites and tree topologies in one B-VPLS instance, thereby ensuring the stability of the backbone network and reducing the network operation cost.