Abstract: The study aims to design a soil nitrogen content sensor based on Frequency Domain Reflectometry, which is one of the theories of soil dielectrics, to provide a novel method for rapid determination of nitrogen content in soil. With two parallel plates as sensitive element, nitrogen soil as dielectric, the experiments were designed to explore the impact of nitrogen content on the dielectric properties of the soil under different soil moisture gradient in the experimental device, and with the aid of the magnetic field effect parallel plate electrodes to detect the nitrogen content in the soil and build the relationship model of the sensor output frequency and soil mass water content and soil nitrogen content. The results indicated that when the soil water content was the same, the output frequency of the sensor with the nitrogen content decreased progressively. The adjust R2 of the model was 0.97 which could demonstrate the validity of the model proposed. On the basis of previous research, by using sophisticated soil moisture sensor based on Standing Wave Ratio, the dielectric content of nitrogen of soil could be indirectly obtained and the soil bulk density could be gained by the drying method, so as to know the accurate moisture content of soil that contained nitrogen, therefore, the soil nitrogen content could be calculated through the model according to the output frequency of the sensor and soil water content.