A robust and efficient key management scheme is necessary for mobile ad hoc networks (MANETs) to ensure its security. CPK (Combined Public Key) is a recently developed public key algorithm, which has advantages comparing with other public key system, such as low storage overhead and calculation demand. Some key management approaches based on CPK have been proposed for MANETs in recent years. However, CPK is vulnerable to collusion attack which may bring about terrible threats. In this paper, a novel key management scheme based on double matrix CPK (DMCPK) is put forward, which is resistant to collusion attack. This key management scheme is described in details, including key generation and updating. Simulation experiments are implemented based on the simulation tool OPNET and show the effectiveness of the proposed scheme.