Abstract: Water use efficiency (WUE) is different among species and regions. Few literatures have been reviewed related to long-term WUE of Platycladus orientalis in mountainous areas of Beijing, China. Tree-ring δ¹³C of P. orientalis was used to determine the long-term variation of annual intrinsic water use efficiency (WUE_i) and its response to environmental change. Combining with quantification of tree-ring width, the relationship between net carbon sequestration and WUE_i of P. orientalis was eventually explored. The results showed that mean annual temperature increased with the increase of time from 1918 to 2013, whereas annual precipitation fiercely fluctuated. Tree-ring δ¹³C decreased and WUE_i increased over time. WUE_i was positively related and more sensitive to air temperature increasing than temperature decreasing. Correlation between WUE_i and fluctuated annualprecipitation was ambiguous, which indicated the precipitation was not the main factor affecting WUE_i. The de-trend tree-ring width of P. orientalis increased initially and then decreased, especially in recent 20 years. According to the correlation between WUE_i and environmental factors, temperature resulted in stomatal conductance (g_s) decreasing, which caused a reduction in evapotranspiration and an increase in respiratory loss, leading to the increase of WUE_i and a down trend in net carbon sequestration and tree growth.