This paper proposes a new mapping method combining GMM and codebook mapping methods to transform spectral envelope for voice conversion system. After analyzing overly smoothing problem of GMM mapping method in detail, we propose to convert the basic spectral envelope by GMM method and convert envelope-subtracted spectral details by GMM and phone-tied codebook mapping method. Objective evaluations based on performance indices show that the performance of proposed mapping method averagely improves 27.2017% than GMM mapping method, and listening tests prove that the proposed method can effectively reduce over smoothing problem of GMM method while it can avoid the discontinuity problem of codebook mapping method.