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For the modern intelligent irrigation system, a moisture sensor with high measurement accuracy, low power consumption, low price, small structure and simple calibration process is urgently needed. A capacitive low-cost soil moisture sensor and its signal processing circuit was presented based on the principle of RC steady-state response peak detection. The characteristic of the sensor circuit was that a RC series circuit realized slow charge and fast discharge by adding a discharge diode. The influence of the sensor’s sensitive area, electrical characteristics, calibration model, temperature drift and conductivity on the output signal was tested. Experimental data analysis showed that the average sensitivity of the sensor for measuring volumetric water content was 12.187mV, and the sensitive area was 3.8cm×2.5cm×7.2cm;the output signal of the sensor was not affected by the supply voltage, and the current consumption was only 3~4mA;by calibrating in different dielectric constant solutions and combining TOPP’s empirical formula, based on working principle the exponential calibration model of the sensor can be well fitted to the relationship between relative dielectric constant and soil volumetric moisture content, and the determination coefficient R2 was greater than 0.96. In the range of 5~50℃, the measurement error caused by the temperature drift of the sensor was about 0.5%. In the range of 0~2000μS/cm, the maximum measurement error of the sensor caused by conductivity was less than 4.2%. The measured data in Loess and cinnamon soil showed that the maximum error of the sensor was 2.17%. Compared with similar sensors, the test accuracy was comparable, and the cost-effective advantage was obvious. It was especially suitable for large-scale wireless soil moisture sensor detection systems.