Two kinds of methods are used to calculate the aerosol optical depths of wide-band from the direct solar irradiation detected by pyrheliometer on the ground under the condition of cloud free sky. The results are obtained to be similar. The analyses of two methods show that the first method is conciser, while the equation used in it is curt, the second one is to definite the opximum wavelength, and the optical depth of the wavelength is regarded as the wide-band aerosol optical depth. So, the latter one is more precise on the theory, and more information can be obtained from the result. In a word, the latter is more resonable than the former.