Abstract: A method for calculating the stability safety factor of slope supported by composite soil nailing is proposed. A formula of stability safety factor is established using the Bishop method. The critical slip surface and its safety factor are found by expressing the position of trial slip surface in terms of a few properly chosen geometric parameters. The slip surface is searched through moving the three geometric control points on the ground surface, the bottom of excavation and the surface at half of the height of the excavation, respectively. Several engineering projects are analyzed using the proposed method. Results show that(a)the critical slip is obtainable and unique;(b)the improved method is more reasonable than previous methods;(c)the hypothesis that the slip surfaces through the foot of slope is unreasonable for vertical slopes in soft ground; and(d)the agreement with the factual results indicate that the present method is of practical value.