Based on the analysis of soil dielectric constant,the measuring principle of capacitive soil moisture sensor is studied. This paper uses ANSYS software to establish a three-dimensional model of a capacitive moisture sensor probe, and studies the influence of the sensor on its capacitance value, electric field intensity distribution, moisture content detection depth and sensitivity when the structural parameters of the sensor probe change. Optimizing size parameters, a capacitive soil moisture content sensor based on frequency domain reflection method is designed. The simulation experiment results show that when the probe ring electrode radius is 28 mm, the axial width is 28 mm, the electrode spacing is 12 mm, and the electrode thickness is 1 mm, the sensitivity, signal measurement strength and detection range of the probe are significantly improved compared with the control product. It is a capacitive soil The structural design of the moisture content sensor provides a theoretical basis.