Abstract: Water is one of the most important factors which restrict China's agricultural production. The test of soil moisture content has the vital guiding significance on agriculture production practice. The proposed sensor only measures specific wavelengths which are sensitive to soil moisture, and does not need a full spectrum of certain wavelengths range scanning. As a result, filters, gratings, optical lens and other optical devices can be omitted. As the radiation area and the scattering angle of the scattered light from samples are very big, in order to guide all light into photoelectric conversion system and get stronger output signal of the reflected light, it is important to collect the diffuse light on the surface of the soil and to design optical unit of the sensor. The sensor used two light sources to irradiate the same position of the soil alternately, and the photoelectric converter was located above the soil surface, with the two light sources located in both sides of the photoelectric converter and the angle between the incident lights of the two light sources and the soil surface to be 45 degree respectively. Based on characteristics of the soil moisture absorption and reflection of different spectrum, light emitting diodes (LEDs) are used as light sources of the sensor, with the measuring light center wavelength to be 1 940 nm and reference light center wavelength to be 1 800 nm. A dedicated measurement circuit and the corresponding software were designed to measure soil moisture content. The light from the light source of the system reached the soil surface, after the reflection, then we assent to amplification circuit, analog-digital converter, display and storage in series. The circuit part included STC15W4K32S2 microcontroller, signal amplification circuit, real-time clock circuit, serial port module circuit, E2PROM circuit, LED driver circuit, LCD display circuit, key interface circuit, reference voltage circuit and A/D converter circuit. The software part included the initialization of internal ADC conversion module, MCU internal timer interrupt mode, LED pulse mode and A/D conversion. After the completion of the sampling and processing, related data was stored and sent to the PC via serial port module. From January to June of 2015, we designed this instrument and did experiments. There were five kinds of soil samples which were black soil of northeast, Yangling (Shaanxi province) loess soil, Yan'an loessial soil, purple soil in Chongqing and Jiangxi Red soil. By measuring a large numbers of samples for voltage transferred from light intensity, relative reflectance decreased with the increase of soil moisture content. The relationship between soil moisture content and the intensity of the reflected light was established by measuring the reference wavelength of the soil surface and the wavelength of the reflected light intensity. So we could establish a linear relationship between soil moisture and relative absorption depth, and the exact coefficient of its linear regression was 0.86. Calculating for each soil sample with relative absorption depth average value, we concluded that there was a positive relationship between the soil moisture and relative absorption depth. In order to prove the accuracy of measure instrument, the actual soil moisture was compared with measurement. The results showed that the error of root-mean-square between the oven drying method and the result of soil moisture content made by this instrument was 3.9%. Physical size of this equipment was no more than 100 mm×8 mm×30 mm, which was easy to carry. According to the small size of the sensors, the overall design requirement of the structure was simple, and the same location of the sensor was considered in the same position of the two kinds of light source. So this design meets the requirements of smart size and high measurement accuracy.