Unfavorable hydrodynamic evolution is considered as the major cause leading to reservoir slope instability and is often modeled by numerical method. However, this simulation is seldom checked by systematic field instrumentation. Taking the opportunity of filling the Three Gorges Reservoir, a system was established in Xietan landslide to monitor reservoir water level, subground water level, seepage pressure, rainfall and deformation, etc. The monitored data during reservoir filling shows that: (1) The water level rise in the bank lags behind the reservoir filling and the lag time depends on the bank permeability; (2) rainfall-induced subground water rise and its lag time is closely correlated to hourly rainfall, indicating that it is not feasible or sufficient to use daily rainfall for analysis; (3) the effect of inverse seepage during reservoir filling on stability is ephemeral and reservoir filling is the major cause leading to bank instability.