A method, a device, and a system for transmitting a multicast packet are provided. The method includes: when a device that is in a source subnet and that is connected to a core network receives a multicast packet of a target multicast group, determining addresses of devices that are in a plurality of destination subnets corresponding to the target multicast group and that are connected to the core network, replicating the multicast packet, to obtain a plurality of multicast packets whose quantity is equal to a quantity of the plurality of destination subnets, separately adding outer encapsulation to each multicast packet, and forwarding the multicast packet to which the outer encapsulation is added, where a destination address in the outer encapsulation is an address of a device that is in each destination subnet and that is connected to the core network.