Soil probes with 4-pin configuration have been widely used in frequency domain (FD) and time domain reflectometry methods for determining soil water content. The techniques of FD sensors largely rely on the port impedance of the probe. This study provided a 3-dimensional numerical model to represent the electric behaviours of the 4-pin probe, which is valuable for analysing the effect of the soil dielectric constant, geometry, and the operating frequency on the port impedance of the probe. The model was performed with high frequency structure simulator software based on Maxwell’s equations and finite element method. A typical 3-dimensional electromagnetic distribution of the 4-pin probe was presented. The model was validated with 3 experiments under the aid of a network analyser. First, the experiment was performed using a series of fluids of known relative dielectric constants, then numerical simulations were carried out and confirmed by soil sample test with varying frequencies and the probe lengths. The effects of these parameters on FD methods are discussed based on the 4-pin probes. The 3-dimensional numerical model appears to be a meaningful tool to investigate more deeply a 4-pin probe in FD method.