The invention discloses a method for establishing a session between a caller and a callee, applying to a next generation network, after said callee picking up, which includes: restarting a session negotiation with said callee via a first application server corresponding to the callee, upon obtaining a session description protocol of said callee, sending an update request message containing a session description protocol of said callee to a second application server corresponding to the caller; renegotiating, by the second application server, session with a caller according to said session description protocol of said callee; after the caller completing session update, returning a response message containing a session description protocol of said caller to said callee. The method can realize the multi-service nesting triggering or the forwarding triggering of several flows in one service, and can guarantee the proper connection between the caller and the callee.