Group key agreement protocol is a effective solution to establish a common session key for communication in ad hoc network. But, due to frequent topology structure changes in ad hoc network, the member changing environment in group communication must be considered, besides establishing a common session key. Combined with complete binary tree architecture, this paper proposes a new identity-based group key agreement protocol, based on ID-AGKA protocol. The new protocol can reduce computing and communicating work, and provide many security properties , such as implied key authentication, known session key security, forward security, against key-compromise impersonation attack, against unknown key-share attack, key control and so on. Further, it has higher performance.