Abstract: ElGamal type threshold signature is an important part of threshold signature. There exists a problem in the ElGamal type threshold signature proposed before that all signer must generate corporately a random number before threshold signature is executed. The computation and communication overhead of the scheme is heavy and not suitable for Ad hoc networks. An improved ElGamal type threshold signature scheme was proposed for Ad hoc networks. The composite public key (CPK) technology was used for generating random number. The scheme is composed of key initialization and threshold signature. All network nodes corporately generate a secret key and a random number matrix using distributed key generation protocol in the key initialization phase and every node hold secret key share and random number matrix share. When issuing part signature every signer select random number from its own random number matrix share using the same arithmetic. Then the part signature is used to reconstruct whole signature. The security of the scheme was proved in random oracle (RO) model. The practicability analysis shows that the computation complexity and the communication cost of the scheme are low, but the efficiency and success ratio are high.