Abstract: The two-dimensional slip surfaces are usually generated by connecting a series of nodal points, of which the coordinates are optimized to locate the critical slip surface. The more the number of nodal points is, the more the number of optimization variables. Based on several nodal points, the Spline function can be used to generate smooth curve as the slip surface. And it is also applicable to the soil slope containing soft layers without pre-definition of lines between two adjacent nodal points. The factor of safety related to the given slip surface is obtained by unbalanced thrust force method and the harmony search algorithm is adopted to determine the critical slip surface for three soils slopes. The performance study on the generation of Spline Function is performed.