Resistance spot welding (RSW) has rarely been applied to the fields requiring high welding quality, as it is limited by the instability of welding quality and quality monitoring system. Inversion imaging of nugget is a new developing method of monitoring welding quality at this background. Before the inversion image, this paper does some researches on the electromagnetic forward problem by establishing the forward model and simulating its electromagnetic field. In order to verify the reliability of this model, this paper takes the magnetic field signals as example to carry out the spot welding experiments. By comparison, the data illustrates that this electromagnetic model is reliable within a certain distance range. Besides these, this paper also does some researches on the different patterns of Hall sensor array, in order to find the array pattern with smaller errors to carry out inversion imaging.© (2010) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.