Abstract: In the measurement system of time domain reflectometry (TDR), three-rod probe is most widely used. The sampling area of this probe form is an important question in engineering application. In this paper, through comparative analysis of Maxwell equations and groundwater seepage equation, the software for groundwater seepage (e.g., the SEEP/W module in the Geostudio) is used to compute the sampling area of a typical three-rod probe. A special experiment is designed to verify the computed results. The results show that: (1) the results of sampling area are well consistent between the numerical computation using Geostudio and the experiment; (2) the sampling area of the three-rod probe approximates an ellipse with semi-major and semi-minor axes of 33.0 and 20.7 mm respectively. In this area, the media with 50% influencing the measured results concentrate around the central rod, and this area also approximates an ellipse with semi-major and semi-minor axes of 10.7 mm and 8.7 mm respectively. It shows the “skin effect” significantly; (3) the electrical conductivity of the media has insignificant effect on the sampling area of the three-rod probe. A computational method for the sampling area of three-rod probe is poposed, and it provides guidance for the design of three-rod TDR probe in engineering and experiments.