A transmission method and system for Converged IP Messaging (CPM) is disclosed, which can create Message Session Relay Protocol (MSRP) links between a CPM client and an originating participating function entity and between an Inter-Working Function entity (IWF) and the originating participating function entity respectively. The CPM client sends CPM message through each MSRP link to the inter-working function entity which sends the received CPM message to a non-CPM system after the protocol conversion of the CPM message. With the help of the method and system, the transmission link between the sender and the IWF is created by the session invite message of the large message mode CPM, and the content of the large message mode CPM is sent to the IWF which then transmits the content of the large message mode CPM to a non-CPM client of the receiver. The method and system resolves the problem that the content of the large message mode CPM can not be transmitted to a non-CPM system receiver in the prior art and improves the user experience.