A multicast data transmission method includes: determining, by a first BFIR in a first BIER domain, a BIFT-id and a bitstring that correspond to multicast data in a second BIER domain, where the BIFT-id is determined based on at least an SI to which a BFR-id of a first BFER belongs and a BSL supported by the first BFER, and the first BFER is a BFER in the second BFER domain that is used to receive the multicast data; encapsulating the multicast data into a BIER data packet, where a BIER header of the BIER data packet includes the BIFT-id and the bitstring that correspond to the multicast data in the second BIER domain; and finally sending the labeled BIER data packet to a second BFIR, where the label is a label corresponding to a prefix of the second BFIR.