Because of the limitation and limited usable range of traditional satellite in-orbit calibration methods, a new in-orbit calibration method based on an aerostat is proposed. In the method, the satellite is calibrated via the solar radiation reflected by an aerostat-borne mirror array. An in-orbit calibration model based on an aerostat is established and analyzed. The result shows that compared with the traditional in-orbit calibration methods, this method has the advantages of short atmospheric path, wide wavelength range and high calibration precision.