A method, a system, and an apparatus for extranet networking of a multicast Virtual Private Network (VPN) are disclosed in the present invention. The method includes: establishing a multicast tunnel to a source VPN multicast instance and a multicast tunnel interface (MTI) connected to the multicast tunnel according to an added Share-Group address of a source VPN; sending an encapsulated Protocol Independent Multicast (PIM) protocol message to the MTI, forwarding the encapsulated PIM protocol message in a public network, and making the source VPN multicast instance generate a public network multicast data message of a Share-Group of the source VPN according to the encapsulated PIM protocol message; and decapsulating the received public network multicast data message of the Share-Group of the source VPN, and obtaining multicast data from the source VPN multicast instance. The present invention reduces the resource occupation of a router system, and alleviates the network load.