Many wireless ad-hoc applications for military battlefield and emergency situations depend upon secure group communications. But group key management for ad-hoc networks meets a great challenge because of mobility, limited power, unreliable multi-hop communication and lack of trusted center. Based on the identity-based authentication mechanism and threshold cryptography, a new identity-based distributed group key management scheme (IBDGK) is proposed. The proposed scheme has the characters of security, robustness and self-adaptability. It can resist coalition attacks and effectively trace the malicious nodes. The results show that, compared with some previous work, IBDGK has notable superiority in performance when group size is approximately no more than 100, and has similar performance for large group.