Stable isotopes of deltaD and delta~(18)O in precipitation are investigated in western mountains of Beijing from July 2011 to July 2012. The results show that the mean values of deltaD and delta~(18)O in precipitation in the study area were lower than those in global and in China, and had great difference to those in Beijing, and the isotope values of precipitation within the same month showed great variations. Because of the mutual influence of monsoon climate and atmospheric water vapor cycle, the stable isotopes of deltaD and delta~(18)O in summer precipitation were less. The isotopic values decreased with the increase of monthly precipitation and temperature from April to July since the rainfall played the leading role in the interaction process of temperature and rainfall. The isotopic values increased from July to September when the temperature took the leading role. There are more stable isotopes of deltaD and delta~(18)O in winter precipitation since the precipitation was formed in the continental air masses from the northerly winds. The second evaporation after the rainfall in the process of atmospheric water vapor cycle also causes the piecewise fluctuated decline of deltaD and delta~(18)O in a typical long duration of rain. The meteoric water line equation of the study areas was deltaD=7.17delta~(18)O+1.46 (R2=0.934 2), and the slope and intercept were lower than those in the national and global atmospheric precipitation line equations. When compared to the meteoric water line equation of Beijing, the slope of the equation was similar and the intercept was obviously different, which indicated that the ocean water vapor was not the only source of water vapor in the study area, the values of isotopes in the precipitation in this area were also influenced by the continental air masses, local strong water vapor cycle and the drying process in recent years. The irregularity of deuterium excess indirectly reflected the complexity of water source in the study area.