An adaptive calculation method for realizing the threshold segmentation algorithm is put forward by using threshold segmentation algorithm based on transitional region as a theoretical foundation.This kind of non-interactive method can not only avoid the manual intervention of choosing threshold value in image segmentation but also gain ideal digital edge of Tooth Contact Zone (TCZ).During the pretreatment process for changing true color image of TCZ to gray image,the adaptive calculation for trichroism coefficients is realized by using the statistics as a foundation and focusing on concrete true color image.The effect of the pretreatment provides entire image information for the subsequent image segmentation.Experiments show that the method developed in the paper for extracting digital edge of TCZ of bevel gear features is applicable.