[Objective] To analyze the effects of litter layer on forest ecosystem moisture cycle. [Methods] Using stable isotope techniques to measure the moisture in Quercus xylem in dry and wet seasons, and the isotopic characteristics of the litter layer and different soil moisture layers. By contrasting the changes of the composition of litter layer and soil moisture isotopes under different environmental conditions which including before and after periods of drought and rainfall, and based on the differences between the changes and the isotopic characteristics of plant stem moisture, the differences in different seasons of cork oak were estimated. [Results] Firstly, as the drought continues, due to evaporation fractionation gradually, the isotopic characteristics of moisture in surface soil of 0-30 cm become enriched for the average thickness of the litter layer, and the isotopic characteristics of soil moisture almost did not change for the thick undecomposed litter layer caused by special topography. The moisture source of cork oak concentrated mainly in the surface water, and did not change with the drought. Secondly, after extreme rainfall during the rainy season, soil isotopic characteristics indicated litter interception rainfall effect was obvious, the rainwater which was trapped by litter infiltrated to soil in the form of plug flow continuously, the main source of moisture of Quercus come from the surface 0-10 cm layer of soil litter decomposition layer. Thirdly, moisture gradient isotopic characteristics of the soil profile was related to the structure of the soil layer. [Conclusion] There are differences in the effects of litter layers' thickness on soil moisture isotopic characteristics, especially which of undecomposed layer. The hydrological effects of litter in cork oak stands change the plant water use indirectly.