Resistance spot welding is an important technology in the field of automobile manufacturing, the quality of nugget determines the quality and strength of body to a large extent. It is, however, irrational to apply the resistance spot welding to the dominations higher requirement to the quality of nugget for the limitation of the quality control. In this paper, based on the electromagnetic theory, the inversion theory is introduced to analyze the spot welding process. First, the magnetic field around the nugget is detected, with which the current density distribution in the welding nuclear can be calculated inversely, where the truncated total least squares (TTLS) method is recommended to solve the inverse problem. The result shows that inverse images can basically reflect the current distribution in the cross section of nugget under different welding currents.© (2010) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.