This study focuses on the method to determine the slip surface in waterfront soil slope analysis under static and seismic conditions. Based on the limiting equilibrium theory and the stress analyzing method, a new method to determine the slip surface is suggested. In the method, the two basic assumptions are considered in order to solve the problem. One is that the slip surface comprises a series of straight lines, and the other is that the interslice boundary is an inclined plane. Three balance equations for any slice are proposed. The iterative method to solve the balance equations is also suggested. In the new method, the slip surface is obtained slice by slice going from downhill to uphill in terms of the balance equations of the slice, not predefined.