The performance of on-orbit payloads is inevitably degraded. Field calibration is a primary method of on-orbit radiometric calibration for some satellite payloads， and its accuracy mainly depends on how accurate the surface reflectance is. This study introduces the working principle and system composition of the Automatic Observation System of Surface Reflectance， and unattended， automated， and continuous spectral surface reflectance measurements were conducted at the Dunhuang Radiation Calibration Field. The short- and long-term stabilities of the surface reflectance data acquired simultaneously were compared and analyzed using the standard whiteboard and irradiance methods， respectively. The surface reflectance is directly traceable to the reflectance of the standard whiteboard， and the data processing method reduces the calibration transfer link. The results are as follows. The mean deviation of short-term measurement of the surface reflectance by the standard whiteboard method is 0.130%. For the long-term measurement， the mean deviation of the 350-600 nm band is 4.996%， and the mean deviation of the 600-2 500 nm band is 2.104%. The standard whiteboard method is reasonable and feasible for measuring the surface reflectance， and the figure of reflectance is continuous and smooth， and it has less fluctuation. The accuracy of reflectance can be improved by regularly cleaning the standard whiteboard. The short-term measurement of the irradiance method is similar to that of the standard whiteboard method； the mean deviation of the 350-1 900 nm band is 0.236%， and the mean deviation of the 1 900-2 500 nm band is 0.443%. For the long-term measurement， the overall drift of the reflectance is small， with an average drift of 0.735%. However， there is a disadvantage of large local noise in the reflectance curve. The two methods can be combined to obtain accurate surface reflectance data.