For slope failures on reservoir, we can’t only see the damage from their deformation and failure, but also think about the disaster from the tsunami caused by them. This paper attempts to obtain the law about spread and attenuation of the reservoir tsunami caused by slope failure. It uses the software of FAST (Fast Assessing System for Tsunami) and GEO-WAVE to numerically simulate such tsunami. It selects the slope #4 at Gongjiafang on the Three Gorges reservoir as the actual case. The simulation area is about 23 km long and 10.4 km wide and the reservoir water levers are 175 m, 156 m and 145 m respectively. Figures about the tsunami are gained with the numerical simulation. The pictures of wave probes are charted with calculation and analysis of data processing module. Comparative analyses of simulation results under the three water levers are given. As water lever decreases, the largest wave height and the largest climbing height have an increasing tendency, but the time of wave damaging the passage gets shorter. The largest wave height has a distribution rule that the shape in middle of river is concave and the shapes on both sides of river are protuberant. The rapid attenuation area of impulse wave distributes in the region at a distance of 1 km from the source point. The higher is the wave height of source point, the more is the drop height of impulse wave in unit length. Because the waves can be superposed near bank, it is suggested that boats must speed through along the middle of the reservoir river immediately when they are near the slope #4.