According to perturbation method in hydrodynamics，the differential equations including continuity equation and movement equation of transient flow in open channel are transformed into polynomials. The constants of polynomials are the values when water level keeps in silence，and the constants and the first differentials are selected to linearize these equations. The decay rule of surge during landslide failure site is gained by variable separation method on basis of taking exponential decay form of surge into account and the superposition of surge height is carried out. Taking Xintan landslide in the reservoir of Three-Gorges as an example，the vertical slice method and the Newton law are taken to analyze the movements on ground surface and in water，and the momentum theory is used to calculate surge height triggered by each slice. Then，in terms of decay formula，the height of surge after superposition in landslide failure site is obtained. It is concluded that superposition method considers dynamic process of the surges when landslide fails，the surge height is triggered by each slice，the surge height after superposition increases first and then decreases.