Abstract: During the operation of a hydropower station diversion tunnel linked with a high slope， the monitored data show that the upper slope has an abnormal deformation tendency towards the upstream， which is different from the general slope deformation law. The deformation mechanism of the high slope linked with the hydropower station diversion tunnel outlet is analyzed in detail based on detailed analysis of the geological data， monitored data and external influence factors. A thousand-year frequency flood in 1998 is an obvious primary factor affecting the change in deformation tendency. At the same time, slope′s structural features of rigidity upwards and weakness doamwards and the fully developed interlayer shear zones are the internal reasons of the slope deformation； excavation unloading of the slope， reservoir impoundment， change in reservoir water level， rain， flood atomization， rock mass weathering， long-term interaction given by water and rock mass must be the other external factors influencing and aggravating the rock mass deformation. At present， the deformation at the basic monitoring points trends towards stability， however the joints， fractures， faults and interlayer shear zones have a very important role in the deformation and stability of the slope. Therefore， it is important to keep the long-term monitoring， and any unusual case must be analyzed and handled in time.