Moisture content is the most direct and necessary index to analyze and judge agricultural drought, realize watersaving irrigation and hydrological forecast. In order to grasp the information of soil moisture content in time and accurately, a soil moisture monitoring system based on narrow band internet of things (NB-IoT) was designed. Circular cylinder was adopted by the probe of the system. The moisture content of soil was measured by the output frequency of the frequency divider in the probe, and the capacitive soil moisture sensor was calibrated. The calibration experiment of the sensor showed that the calibration curve adopted the cubic polynomial curve of the output frequency of the sensor, and the correlation was good, and the determination coefficient can reach 0.9851. The influence of environmental temperature change on the measurement accuracy of soil moisture content was analyzed with a thermostat, and the temperature change of 1℃ and the frequency division of the probe was 0.1386kHz, a first order linear equation was obtained to compensate the influence of ambient temperature on the frequency output of the sensor. Soil moisture content was measured at 50.4~65.5℃ and -18.0~23.6℃. The results show that the system can run stably and reliably at high temperature and low temperature.