This　paper　proposes　a　space–time　Trefftz　method　(STM)　to　study　the　seepage　mechanism　and　stability　of　unsaturated　slopes.　The　groundwater　flow　under　transient　conditions　is　important　in　engineering　practice　for　solving　practical　problems　such　as　assessing　the　stability　of　unsaturated　soil　slopes.　Based　on　the　transient　groundwater　equation,　we　derived　the　Trefftz　basis　functions　by　splitting　the　variables.　The　solutions　are　approximated　using　Trefftz　basis　functions　in　the　space–time　domain.　The　Stabl　software　is　subsequently　employed　to　analyze　the　stability　of　the　slope　under　the　rainfall　recharge　condition　with　the　combined　reservoir　water　level　fall.　The　results　demonstrate　that　the　steeper　the　hydraulic　slope　drop　under　combined　reservoir　water　level　fall　and　rainfall　infiltration,　the　more　unstable　the　slope　becomes.　©　2023,　The　Author(s).