In order to accurately grasp soil moisture content information, the problem of real-time observation of soil moisture change in different crop root areas under farmland environment is difficult to observe.The structure of probe was researched and analyzed in detail by modeling simulation analysis, and the optimal sensor structure was determined through experimental verification. Through establishing the trace amount planar capacitance probe two-dimensional model, the influence of change of sensor probe structure on the change of electric field intensity and the capacitance value around the trace amount planar capacitance probe was analyzed by using the finite element analysis, and then the probe structure was determined. The sensitivity and the detecting depth of the probe was optimal when the outer diameter of the probe copper ring electrode was 40mm, the inner diameter was 38.4mm, the axial length was 20mm, and the axial spacing was 15mm. The sensor probe can be applicable to various types of soil moisture measurement. The measurement accuracy reached ±1.42% with better stability and consistency. The results showed that the sensor probe can be designed according to the actual measurement depth and satisfy the soil moisture measurement requirements of the root zone of different crops.