Abstract: This paper calculated coordinate time series using approximate 23-month continuous GPS short-baseline observations, analyzed the effect of multipath on linear trend and annual signals in time series, and determined the long-term trend of Xilongchi Upper Reservoir. Results verifies the overall stability of Xilongchi Upper Reservoir (located in shanxi province) after first impoundment. However, minor linear deformation was detected on main and auxiliary dam, where station L022 on main dam moves at 2.2 mm/yr in the NE 80° direction, station S071 on northern profile of main dam moves at 2.5 mm/yr in NE109°, station L132 on auxiliary dam moves at 0.7 mm/yr in NW87°, horizontal velocities of station S171 and S191 are less than 0.3 mm/yr. And all stations subside at different degree, where station L022 and L132 subside at 3.8 mm/yr and 1.4 mm/yr, respectively, and rest stations subside less than 1 mm/yr. All stations behave annual movement in both north and east component with amplitude of 0.5 ~ 1 mm. The annual movement of staion L022 and TN01 was possibly cause by the thermal expansion of bedrock, and these of rest stations are both effected by bedrock and surface thermal expansion.