The invention relates to a method for realizing multicast between different IP domains and a boundary gateway router. The method at least comprises the steps that: the boundary gateway router acquires multicast group information from an RP router of each IP domain; when the presence of a cross-domain multicast group is acquired according to the multicast group information, and the boundary gateway router sends, via the RP router of the IP domain for a multicast source of the cross-domain multicast group, a joining group request for joining the cross-domain multicast group to the multicast source in order to join the cross-domain multicast group; the RP router of the IP domain for the multicast source forwards multicast data sent by the multicast source; the boundary gateway router receives the multicast data and converts a multicast source address in the multicast data into a target IP domain interface address on the boundary gateway router or a vacant address in a preset target IP domain address pool; a group address in the multicast data is converted into the group address of a target IP domain; and the multicast source address and the group address-converted multicast data are sent to multicast routers interconnected directly in the target IP domain.