A real-time soil moisture measurement method on line scale was proposed and a real-time detection of plant root zone soil moisture information sensing system based on frequency domain oscillometry was designed. The system consisted of soil moisture sensor, PVC casing, electric traction system and controller. The sensor was reciprocated in the casing which was dragged by motors, and it can obtain 240cm real-time soil volumetric water content information. The dynamic response time of sensor was 32ms, standard deviation of stability test was 0.0061V, determination coefficient between measured results and TDR was 0.989, and the requirement of the soil moisture measurement was satisfied. The results of field tests showed that the mean squared error compared with that of BD-III soil moisture sensors based on SWR (accuracy of ±2%) which were buried 10cm apart and paralleled to the sensing system was less than 0.5% when the sensing system was buried under the ground of 30cm. The measured data can reflect the volumetric water content of plant root zone under different rainfall conditions and verify the effectiveness of the system. 