The vertical slice method is generally adopted for calculation of slope stability safety factor. The algebraic expression of the slope stability safety factor Fs is deducted based on the idea of the calculation via the inclined slice method and according to the inclined slice force limit equilibrium conditions. This expression is implicit so that the result can be obtained quickly only through simple iteratives, guaranteeing the convergence. Calculation examples show that the slope stability safety factor Fs derived from this method is very similar with that from conventional methods. which proves that this method is feasible and brings convenience to technical personnel of slope engineering.