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Soil dielectric property is an important physical parameter affecting the measuring accuracy and adaptation of moisture content by using FDR sensor. In order to obtain the best frequency of 7-probes FDR with length of 28mm and needle-to-needle spacing of 10mm, scattering parameters of LF-UHF wave bands (1MHz~3GHz) in four kinds of soil were measured by using VNA. Though transformation based on Logsdon model and Laird model, four soil apparent dielectric permittivities were calculated and figured in Matlab. The mechanism of soil polarization was analyzed by soil dielectric spectrum analysis. The relationship between soil dielectric property and frequency was discussed. The working frequency of 44~398MHz for FDR sensor was confirmed. Taking temperature into consideration, the ideal working frequency band was 62~110MHz in which 75MHz was the best. The soil dielectric value was only sensitive to soil moisture content under the best frequency. It was the ideal frequency for eliminating the effect of temperature and other factors on FDR sensor.