A probe based on standing wave ratio (SWR) to measure stem water content was presented. Two experiments were taken to study the configuration of probe: (1) Simulation experiment. Different organic solutions were chosen as the testing media to simulate the stem which contains water. The result shows that the output voltage changed lineally with the length change of the probe. (2) Calibration experiment. The sample of arborvitae was selected and the oven temperature was kept at 60℃ constantly. The result shows that the relationship between water content of stem and the output voltage was linear. It can be concluded that the probe we studied could be used to measure stem water content. 