For the profile deviation of a stage mirror itself and the profile change caused by the alignment of 2D stage position, an online detection method for the profile deviation of a nano precision 2-D stage mirror was presented. The basic principle of detecting profile differential data with two interferometers was proposed. The influences of zero error and integral error on the measurement were analyzed and a method to improve the measuring precision was put forward. Two sets of original data of the mirror profile were obtained by two sets of detectors with different spans composed by three interferometers. Through the relationship between the two sets of data, the detail profile deviation between the spans was modified. The theoretical calculation, simulation and experiments for the proposed method were performed, and obtained results were compared with that of off-line measurement with a Zygo interferometer. The results show that the most difference in test results is between ±10 nm, and the trend has better consistency, which verify that this measuring method could measure the profile deviation of stage mirrors correctly and could restore its surface errors truly.