The shortage of fossil fuels and environmental pollution have promoted the rise of renewable power generation. The solar energy is one of the famous renewable resources. The defect detection of photovoltaic (PV) panels is of great significance to improve the power generation and the economic operation of PV power plants. At present, few studies focus on the relationship between the surface magnetic field and the internal current distribution of PV panels. Some related studies remain in the qualitative characterization of the magnetic field and current distribution, which lack quantitative assessment and comprehensive investigations. Based on the intrinsic connection between the surface magnetic field and the internal current of PV panels, this article proposes a current distribution reconstruction and busbar current estimation method to achieve the quantitative analysis of internal current and the defect detection of PV panels. The spatial filtering and regularization methods are introduced for reconstructing the internal current distribution of PV panels. The feasibility of the two methods for the current distribution reconstruction and busbar current estimation is verified through the finite-element simulation. A probe for the magnetic field measurement is designed, and an experimental platform is built for the positive current excitation experiment and illumination experiment under laboratory conditions. The panels with a breakpoint in the busbar and two partially covered areas are used to investigate the performance of the proposed method. The ratio of the current in the busbars is proposed as the criterion for the defect detection of PV panels. Both simulation and experimental results verify the performance of this method.