The present invention provides a method of establishing a media transmission link for transmitting a large message mode Converged IP Messaging (CPM) message to a group. The method comprises the following steps: Session Description Protocol (SDP) media parameter negotiation is accomplished through session invite messages and success response messages among a transmitter CPM client, a source participation function entity, a controlling function entity, a destination participation function entity and a receiver CPM client; according to the negotiation result, media transmission links are respectively established between the transmitter CPM client and the source participation function entity, between the source participation function entity and the controlling function entity, between the controlling function entity and the destination participation function entity, and between the destination participation function entity and the receiver CPM client. The present invention solves the problem that there is a lack of the MSRP transmission link in the large message mode in the prior art.