Recognition system based on skin color is main method of character detection of face and hand. This paper analyzed some common method of transformation of image enhance on the basis of YCbCr skin color model. A enhance method which is integrated exponential and logarithm transformation is proposed. And according to the shortcoming of fixed threshold, an automatic threshold algorithm with the fisher estimate function is proposed. The experiment with this method proves that it can enhance the contrast of foreground and background, and has good robust. The algorithm in paper is more adaptive in the recognition and detection than the fixed threshold method.