The present invention provides a method and system for transmitting a large message mode CPM message. A MSRP link is created respectively between a CPM client at a sending end and a participating function entity at a source end, between the participating function entity at the source end and a CPM control function entity, and between the CPM control function entity and an inter-working function entity based on a media negotiation process before the CPM client transmits a large message mode CPM message to a non-CPM system receiver in a target group; then, the CPM client at the sending end sends the contents of the large message mode CPM message to the inter-working function entity through each created MSRP link sequentially; and the inter-working function entity sends the received contents of the large message mode CPM message to the non-CPM system receiver after performing protocol conversion on the received contents. The method and the system solve the problem that a large message mode CPM message cannot be transmitted to a group in the prior art and the user experience is improved.