A detailed analysis was provided based on the port impedance theory. The relationship between probe length and impedance characteristics of soil moisture sensors as terminal open or terminal short was deduced. With the aim to verify the theoretical conclusion, the simulation method by using high frequency structure simulator software (HFSS) and the experiment associated with several organic solutions as references of known dielectric constants were conducted. After a number of experiments, the result showed that the measuring range of soil moisture sensors as terminal open or terminal short, when probe length was within 0~8.3cm, could be up to 0~100%.