Earthquake shakes the soil and induces modulus softening and liquefaction of non-cohesive soil, which causes instability of the slope. A practical numerical method is proposed for predicting large deformation and horizontal displacement of submarine gentle slope due to earthquake. The new method take normal wave loads as pseudo-static steady pressure and initial excess pore water pressure. Using 2-D nonlinear effective stress finite element method, dynamic response and liquefaction analysis are conducted with an infinite submarine slope model. Regarding the seismic motion of soil skeleton as process of softening gradually, softened soil modulus is attained during earthquake. Deformation analysis is performed with the modulus of liquefied and softened soil and the process of lateral movement is obtained. Compared with other related researches, the proposed method shows that gives reasonable results for the conditions indicated. The influences of non-liquefied surface layer, liquefied layer thickness and slope angle on horizontal displacement are studied by series of cases.