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.