Aiming at the problem that the state-of-the-art geometric calibration methods have a long update period and low efficiency, a real-time calibration method that applies the optical auto-collimation principle to on-orbit monitoring is proposed. By installing collimated light sources, area-array CCDs, prisms, and other devices inside the satellite load system, changes in the camera parameters are converted into changes in the spot images. By extracting and processing the spot images, the variation of the camera parameters can be solved to achieve a fast and efficient on-orbit monitoring. The feasibility of the proposed method is verified by building an experimental platform, and the monitoring accuracy of parameters in the pitch and roll directionsis are less than 1″, and the measurement error is better than 0.1″.