Since the nucleation of resistance spot welding is a close and invisible process, it is difficult to monitor its nucleation quality reliably. In order to estimate the quality of weld spot, this paper uses magnetic flux density around the nugget to reconstruct the current distribution in the faying surface between two workpiece. In the study, we found the inverse model of welding current and its magnetic flux density around the nugget. Since the model had ill-posedness, the regularization method was required to solve the problem. Conjugate Gradient Method was used to reconstruct the current distribution. The result shows that the image can reflect the main characteristic of current distribution, and it can be used for estimate the quality of weld spot.