A new calibration method of absolute spectral irradiance responsivity of sun radiometer named CE318 is described. In this method, a tunable laser and a standard irradiance detector which can be traced from the cryogenic absolute radiometer are used as source and transferring standard, respectively. In the experiment, to acquire the uniform, stable and unpolarized irradiance field, the laser which is stabilized by a laser power controller, wavelength by wavelength, is steered into an integrating sphere with which a speckle-removal device is equipped. By the vicarious calibration technique, the spectral irradiance responsivities of the 870 nm sun channels of CE318 consisting of three polarized channels (P1, P2 and P3) and an unpolarized channel (UP) are obtained and used to predict the top-of-the-atmosphere signals, respectively. In the last, the uncertainties of calibration are evaluated and reach to 1.83%, 1.98%, 1.73% and 1.2%, which are comparable to the counterparts of Langley method, the main method of calibration in this field.