A　3D　stability　analysis　is　essential　in　slope　designs,　since　it　is　more　in　line　with　real　situation,　and　can　provide　more　reasonable　results　than　a　2D　analysis.　A　numerical　model　termed　as　ISR3DNMM-GPS　method　is　proposed　for　factor-of-safety　(FOS)　calculation　and　3D　failure　surface　(or　critical　slip　surface)　determination　involved　in　a　3D　slope　stability　analysis.　In　the　proposed　numerical　model,　an　improved　strength　reduction　technique　(ISRT)　which　can　avoid　unreasonable　plastic　zones　appearing　in　the　deep　area　of　a　slope　is　implemented　into　the　3D　numerical　manifold　method　(3DNMM),　and　an　improved　strength-reduction-based　3DNMM　(ISR3DNMM)　is　developed.　With　the　proposed　ISR3DNMM,　the　FOS　and　generalized　plastic　strain　(GPS)　field　corresponding　to　slope＇s　failure　state　can　be　obtained　simultaneously.　Then,　a　group　of　useful　3D　points　locating　within　the　plastic　zone　of　the　slope　are　further　determined　based　on　the　3D　slope＇s　GPS　field.　With　these　3D　points,　an　improved　least　square　method　is　adopted　to　obtain　a　rational　3D　failure　surface,　which　passes　through　the　slope＇s　weakest　point.　With　the　proposed　ISR3DNMM-GPS　method,　the　stability　of　two　typical　3D　slopes　are　analyzed.　The　numerical　results　indicate　that　3D　slopes＇　stability　can　be　effectively　analyzed　with　the　proposed　numerical　model.　The　proposed　numerical　model　deserves　further　investigation.