Identity-based proxy signature enables an entity to delegate its signing rights to another entity in identity-based cryptosystem settings. However, few existing scheme has been proved secure in a formalized model, or acquired optimized performance. To achieve the goals of both proven security and high efficiency, this paper proposed an efficient identity-based proxy signature scheme. The scheme is constructed from bilinear pairing and proved secure in the random oracle model, using the oracle replay attack technique introduced by Pointchval and Stern. The analysis shows that the scheme needs less computation costs and has a shorter signature than the other schemes.