Image preprocessing is the pioneers for continuing to conduct image-related studies at late time, and accurately and effective using proper appropriate pretreatment methods plays the key role. After deep study on Bayesian formula and its related theories,according to the characteristics of high-efficient mega-data processing and strong independent object processing of Bayesian algorithm, overall threshold and piecewise threshold are set and the optimal threshold segmentation method is searched,as a result, human face image is preprocessed. Simulation experiment shows that the obtained conclusion indicates that this method can take advantage of short time to clearly distinguish human face and its organs and shield outside influence.