After concisely analyzing the typical group key agreement protocols of centralized key distribution (CKD),Diffie-Hellman (DH) and group Diffie-Hellman (GDH), tree-based group Diffie-Hellman (TGDH), the paper gives out a new key management method by threshold cryptography and tree-based structure, which is proved to gain advantages in computation cost greatly and improves the MANET key management performance accordingly.