In order to quantitatively study the migration pattern and the degree of contamination of LNAPL in different water distribution characteristics after leakage, this paper employs Electrical Resistivity Tomography (ERT) to monitor the migration pattern of diesel oil in a three-dimensional sandbox under varying water distribution conditions. The experimental results demonstrate that it is possible to estimate the diesel saturation degree by establishing a relationship between diesel-water resistivity and substituting the acquired resistivity and water content. In stratified water distribution characteristics, the contamination range is distributed in the form of a “volcano”. In uniform water distribution characteristics, the phenomenon of water-oil displacement occurs, resulting in the accumulation of water in the center region. In the case of uniform water distribution, the displacement of water by oil results in the accumulation of water in the center area, which forms the dominant channel of LNAPL. The final distribution of the contaminated area is C-shaped. The findings of this study serve as a valuable reference point for further investigations into the migration patterns of LNAPL and the extent of contamination.