A kind of method with using of vertebra medical image to deal with image key data was presented. First, used of the method of piecewise linear transform to stretch gray level of image for enhancing contrast ratio between target area and background area or reducing the detail in the target area. Second, used of information of pixel gray level to set up weighing chart, choose target seed pixels and background seed pixels, and then make use of rand-walk algorithm to divide up image so that receive edge of contour line. At last, unit area pixel average gray-level in the range of edge contour line was computed. Through comparing gray value of unit area in the region with bone density which can be gotten from physic testing, the bone density may be received in the checked region.