The determination of soil moisture content is of great significance for agricultural irrigation decision-making, crop yield improvement, effective management of water resources, and the study of soil mechanical properties. The open-ended coaxial probe method measuring the dielectric of soil was used to characterize the moisture content of soil, which is simple and accurate. Based on the open-end coaxial probe method, a quasi-static mathematical model was established for the measurement system with a short-circuit board. The appropriate thickness to be measured was selected to make the model suitable for the dielectric measurement of semi-infinite soil. Through the comparison of the full-wave simulation and model calculation results, the average absolute error of the real and imaginary parts were 0.001 and 0.022 respectively. Comparing the measured and theoretical values of the absolute ethanol dielectric, the average absolute errors of the real and imaginary parts were 0.375 and 0.279 respectively to verify the accuracy of the model. The model was used to measure and calculate loess soils with different moisture contents. The second-order polynomial fitting determination coefficient of real part of the obtained complex permittivity and the measured soil moisture content was greater than 0.965, indicating that the soil dielectric measurement model proposed was suitable for soil dielectric measurement of electrical value and moisture content.