Rainfall　infiltration　can　alter　the　pore-water　pressure　and　decrease　matric　suction,　which　can　lead　to　slope　instability　and　landslides.　Most　of　the　previous　numerical　studies　conducted　on　rain　infiltration　have　assumed　a　two-dimensional　(2D)　failure　mechanism,　which　neglects　the　three-dimensional　(3D)　characteristics　of　slopes.　An　analytical　assessment　framework　was　developed　using　the　upper-bound　theorem　of　limit　analysis　to　determine　the　factor　of　safety　and　failure　pattern　in　the　case　of　a　3D　vertical　cut　slope　under　diverse　rainfall　situations　combined　with　the　unsaturated　soil　mechanics.　The　Green-Ampt　model　was　adopted　to　depict　the　infiltration　process　and　the　progression　of　the　wetting　front　into　the　soil.　The　slope　factor　of　safety　was　determined　using　the　energy　balance　equation　by　adopting　the　strength　reduction　approach.　The　validity　of　the　framework　was　verified　using　the　numerical　simulation　method.　The　effects　of　duration,　intensity,　and　patterns　of　rainfall　and　the　3D　traits　of　slope　on　the　slope　failure　pattern　and　stability　were　determined.　A　set　of　design　charts　was　included　for　the　preliminary　design.　The　present　study　quantitatively　determines　the　stability　of　slopes　susceptible　to　rainfall　infiltration.