The soil dielectric spectrum are related to the soil physical properties, and those are affected by the structure of the probe too. In order to determine how the probe structure influence the soil dielectric spectrum measurement, the two kinds of soils were measured at four water content level by 10mm and 14mm spacing needle at 28mm probe length, 28mm and 45mm length needle at 10mm spacing. It has been confirmed that the probe spacing has no effect on the soil dielectric spectrum measurement above 31.2MHz by comparing the dielectric spectrums, and the length of the probe can affect the real or imaginary part of the soil dielectric spectrum in the range of 0.001~3GHz. The research shows that the spacing of needle is not the key factor to the soil dielectric property measurement within a certain spacing, and the length of the probe has a greater influence on the soil dielectric property measurement. The smaller of the probe length, the larger of the dielectric relaxation frequency. Therefore, the probe length is the important factor that affect soil dielectric measurements. The FDR probe should be shorter, it is good to improve the measurement accuracy and environmental adaptation.