The mathematical model for a ring-shape four-electrode probe was established and the sensitivity distributions of the electrical potential and the current density in radial and axial directions were calculated by using numerical method under the additional assumptions that the sensor was surrounded with a homogenous medium. By studying the sensitivity field, the measuring range of this sensor was estimated to be an ellipsoids whose radial diameter was about 1.5 times of the spacing between the two current electrodes. Moreover, for the purpose of measuring soil salinity, we studied the relationship between soil apparent conductivity ECa, soil water content θ and soil salinity EC1:5 and fitted an accurate empirical model for converting the ECa detected by the probe to customary EC1:5. This four-electrode probe would provide a simple, quick and inexpensive method to measure and survey soil salinity.