Disclosed in an embodiment of the invention is a service carrying method, comprising: performing, by a communication terminal, and according to a service request, media negotiation with a communication peer in a first access network; performing, according to a media negotiation result, a first media resource reservation in the first access network; if a resource reserved in the first access network does not meet a target resource requirement, performing a second media resource reservation in a second access network; after transmitting, by the communication terminal, a first media resource reservation completion message to the communication peer, receiving, by the communication terminal, a second media resource reservation completion message transmitted by the communication peer upon completion of the media resource reservation; and establishing, by the communication terminal, a service connection with the communication peer, and carrying a media stream in the first access network and the second access network, respectively, according to a carrying method for the media stream corresponding to a target service indicated in the first media resource reservation completion message. Also provided in the embodiment of the invention are a communication terminal, a control element S-CSCF and a system capable of effectively sharing network load, increasing service quality, and improving user experience.