The invention is suitable for the technical field of medical image processing and applying, and provides a method and system for obtaining the value of the bone mineral density of the human body. The method includes the following steps: obtaining the high-low-energy image X ray injection strength, obtaining calibration parameters, collecting low-energy X ray images and high-energy X ray images of the bone area, preprocessing the collected images, receiving an interested area which is selected by a user and requires calculation of the value of the bone mineral density, locating an accurate area of the interested area which requires the calculation of the value of the bone mineral density, and calculating the value of the bone mineral density according to the Beer-Lambert law. By means of the method and system, dependence on hardware of a bone mineral density instrument can be reduced while the value of the bone mineral density is calculated; on the basis of a common digital X ray machine, the scheme of the method and system is applied, measurement and calculation of the bone mineral density of the human body can be achieved, the problem in the hardware is not required to be considered, and the production cost of the bone mineral density instrument is reduced to a certain degree. In addition, when the scheme is adopted for collecting the bone images, a standard body is not required, and the image collecting process is accordingly simplified.