In this study, diesel was selected as a pollutant to study the migration and distribution rule of light non-aqueous phase liquid (LNAPL) in the simulated vadose zone. Saturation was regarded as a critical parameter to reflect the LNAPL migration and distribution rule. To get LNAPL saturation distribution figures, an image processing method of saturation was established to improve light-transmission technology, which can deal with digital camera images. Results showed that the vadose zone contains three areas from top to bottom, named dry media, transition zone and capillary zone. The system has two interfaces at which moisture content increased significantly. The significant increase in moisture content induced two apparent horizontal LNAPL diffusions in the two interfaces. Furthermore, the highly saturated LNAPL was mostly distributed near the wet interface, which lay between the dry media and the transition zone. Moreover, the downstream expansion of LNAPL in the capillary zone was promoted by groundwater flow, yet cutting off LNAPL supply could stop the downstream expansion after a period of time. The accuracy of this image processing method of saturation was verified by mass balance theory and reported a relative error of 4.38%.