Forest floor moisture measurement plays a significant role in the research of forest hydrology, carbon cycle, forest fire spread and risk evaluation. A detection method of forest floor water content based on standing wave ratio (SWR) was introduced. The developed SWR sensor with two parallel anular probes could measure the water content and electrical conductivity in forest floor or mineral soil by using time-sharing multiplexing technology. Experimental results showed that the relationships between the output voltage of SWR sensor and the water content of partly decomposed forest floor, well decomposed forest floor and mineral soil were linear, respectively. Compared with the commercial TDR sensor, the SWR sensor showed a better performance with smaller mean absolute error and lower cost in the measurement of forest floor and soil water content. In addition, the SWR sensor had obvious advantages when the forest floor layer was thin (<10cm). It was notable that the partly decomposed forest floor might be decomposed completely in the long-term in situ moisture measurement. Therefore, the developed sensor estimated the decomposition rate by detecting electrical conductivity of forest floor, which would lay a foundation for the accurate measurement of forest floor water content.