Abstract: Directing to the weakness of the present fixed values mapping methods(method_F),a vocal tract systen conversion method based on the universal background model(UBM) is proposed for improving the performance of the speech conversion system from Chinese whispered speech to normal speech.For the numerous components of UBM the errors produced by the acoustical probability density statistical model can't be ignored.Thus an effective Gaussian mixture components chosen method based on the posterior probability summation of the minimum spectral distortion is developed to optimizing the system performance.The proposed method(method_U) is analyzed and compared using the performance index(PI) based on Itakura-Saito spectral distortion measure.It is shown experimentally that the performance of method_U is more stability for different speakers and different phonemes than that of method_F.The average PI of method_U is better than method_F.It is shown that by selecting effective Gaussian mixture components the PI of method_U can be further improved 5.11%.Subjective auditory tests also show that the proposed method can improve the definition and intelligibility of conversion speech.