Current proxy signature schemes are not proved for their security in the complete provable security model of proxy signature. In this paper, we show a complete provable security model for proxy signature based on Boldyreva's provable security model, and a new identity-based proxy signature scheme are proposed in the standard model, which is based on Paterson's scheme. In the complete provable security model for proxy signature, the new scheme is proved to have the existential identity-based proxy signature unforgerability under an adaptive chosen message attack, which has a security reduction to CDHP. Comparing with other proxy signature schemes based on public key cryptosystem in the standard model, the concept of the identity is introduced into the new scheme, and the new scheme is more secure. © 2012 ISCAS.