Dutta and Barua presented a group key agreement protocol (DB) in dynamic scenario, two flaws have been found by Teo et al.and Tan et al., that is, the leave procedure of the protocol does not provide forward secrecy and the protocol is insecure against an outsider adversary.In this paper, a new scheme is proposed based on DB protocol to avoid the above attacks, and then we prove the security of the improved scheme using extended strand space theory under the Decision Diffie-Hellman assumption.