Aiming at the problem that the position of the light source cannot be determined in practical production, an on-line position calculation for the light source was proposed, which fully analyzed the gray-scale variation of the feature region in consecutive frames. In this approach, a featured region was first selected based on the local gray-scale distribution and used as a reference point, then its position was tracked in the following consecutive frames via block matching method. Combing with the analytic calculation from the light transport model, the relationship between the gray-scale value and geometric position was established. Finally, the set of equations were solved by linear regression method to find the position of light source. The experiment results show that the errors between the calculated results and the measured results are within 5%. The proposed approach has been successfully applied in the real-time manufacturing with good estimation accuracy.