The disclosure discloses a method and system for transmitting a large message mode CPM message, both of which enable the respective establishment of Message Session Relay Protocol (MSRP) links between a CPM client of a sender and an originating participation function entity, between the originating participation function entity and a terminating participation function entity, and between the terminating participation function entity and an inter-working function entity, wherein the establisher of each segment of MSRP link obtains the MSRP connection parameters of the opposite end through media negotiations and establishes the MSRP link according to the MSRP connection parameters; the CPM client of the sender sends the large message mode CPM message to the inter-working function entity through each segment of established MSRP link in sequence, the inter-working function entity performs a protocol conversion on the received the large message mode CPM message and then transmits the converted message to a non-CPM system. The method and system of the disclosure enable the transmission of a large message mode CPM message to an unavailable CPM system user, thus improving user experience.