Electrode-based through-the-earth (TTE) communication system provides an effective solution for information transmission in strongly shielded underground environments. The transmitter′s load impedance determines the magnitude of the current injected into the earth and is a key factor affecting the system′s transmission performance. However, under fixed-voltage excitation conditions, existing through-the-earth communication transmission models typically analyze system performance starting from the current injected into the formation, lacking a quantitative model of the transmitter-side load impedance, which makes it difficult to guide practical system design. To address these issues, this paper first develops a load impedance model consisting of electrode contact impedance, wire impedance, and earth impedance. Based on the physical mechanisms of each impedance component, equivalent circuit models for wire impedance and electrode contact impedance are proposed, along with corresponding mathematical expressions. By measuring the overall system load impedance, the earth impedance is separated and extracted, and a nonlinear regression model between earth impedance and soil conductivity is established, revealing that the earth impedance decreases monotonically with increasing conductivity. Finally, experimental validation of the proposed model is performed under different electrode geometries, soil conductivities, and transmission frequencies. The experimental results show that the predicted values of the model have good agreement with the measured data, with the average relative error maintained within 8%, and the load impedance is insensitive to frequency variation under extremely low-frequency operating conditions, verifying the rationality of neglecting the reactance component. This study fills the gap in quantitative modeling of load impedance for TTE communication systems, provides a theoretical basis for understanding the transmission characteristics of TTE communication systems, and offers a reference for electrode structure optimization and related system parameter configuration.