Combined public key and threshold cryptography are applied to key management for Mobile Ad Hoc Network(MANET) in this paper. The Key Distribution Center(KDC) is used for the initialization of the whole network. In the operation phase, a scheme is proposed to dynamically change the number of server nodes and the threshold value. The method about how to deal with the new node addition and the node eviction is given. The key management scheme is secure, efficient and flexible, so it is especially suitable for the characteristics of MANET.