This application provides a multicast data packet processing method and a forwarding device. The method implemented by a first forwarding device includes: establishing a segment routing header SRH-based tunnel with a third forwarding device, wherein the SRH-based tunnel passes through a second forwarding device that does not support bit index explicit replication BIER; generating a first multicast data packet based on a multicast data packet from a multicast source and the SRH-based tunnel, wherein the first multicast data packet includes a first packet header, a second packet header, and the multicast data packet from the multicast source, the first packet header includes an SRH header, and a destination address included in the second packet header is an address of the second forwarding device; and sending the first multicast data packet to the second forwarding device through the SRH-based tunnel.