Abstract: The three-dimensional simulation of internal charging of complex dielectric structures can help to find the special charging point with distorted enlarged electric field due to irregular boundary condition. In this paper, the three-dimensional internal charging simulation was presented with experiment verification. A three-dimensional simulation was performed to study the internal charging of a typical printed circuit board which is partly grounded on the front surface. The characteristics of electric field distortion emerged on the edge of partly grounded areas were investigated, and the quantitative analysis was carried out considering the dependence of the fillet radius of the metal circuit corner on distorted peak electric field. Under the worst-case of electron radiation environment in geostationary orbit, the results from a 3 mm PBC with 2 mm aluminum shielding show that electric field distortion indeed exists just on the edge connecting grounding and insulation boundary areas. This distorted electric field can reach the level of 10.7 V/m, which is about two orders larger in magnitude than that with uniformly grounding boundaries in one-dimensional case. Enlarging the fillet radius of metal trace corner can reduce the leakage current collection around the grounding boundary, so the peak electric field is reduced. These conclusions can be useful references in effectively diminishing the internal charging hazards of PCBs in a satellite.