The multi-pin soil probe plays an important role in determination of soil water content with a variety of instruments,such as TDR(Time Domain Reflectrometry),FD(Frequency Decomposition) and SWR(Standing Wave Ratio).Therefore,it is critical to calculate the impedance of the soil probe in order to improve the measuring accuracy of these instruments.However,the electromagnetic characteristics of the multi-pin probe is too complex to describe due to the hard-to-define boundary conditions of Maxwell equation.Based on the theory of impedance transform of transmission line as well as a mathematical inequality from the parallel and coaxial transmission lines,a mathematical formula for the impedance of the soil probe was developed.By computing this formula,it was found that the impedance of the soil probe depends not only on the permitivity of soil-water-system,but also on length of the probe-pin and frequency of its measuring signal.Especialy,it is proved that the impedance characteristics of the soil probe may be turned into an inductance from a capacitance and vice versa as measurement conditions change.In addition,it was confirmed that the formula for impedance of the soil probe had a series of periodical disconnected dots.