Due　to　the　vertical　flexibility,　three-dimensional　(3D)　seismic　isolators　have　more　prominent　stability　problems　than　horizontal　isolators.　However,　existing　mechanical　models　mainly　focus　on　the　P-　Delta　effect　(or　axial-load　effect)　of　bearings,　while　less　attention　is　paid　to　the　rocking　effect　of　3D　isolators.　In　this　study,　based　on　the　Koh-Kelly　model,　a　mechanical　model　reflecting　both　P-　Delta　and　rocking　effects　is　proposed　for　3D　isolators.　Then,　a　simplified　analysis　method　of　3D　base　isolation　systems　(3D-BISs)　is　established　and　the　seismic　response　of　a　3DBIS　with　a　large　aspect　ratio　(i.e.,　the　ratio　of　structural　height　to　structural　width)　is　explored.　The　results　show　that　the　3D-BIS　can　reduce　both　horizontal　and　vertical　seismic　responses　of　the　superstructure,　while　the　horizontal　base　isolation　system　(H-BIS)　is　found　to　significantly　amplify　the　vertical　acceleration.　The　rocking　effect　results　in　a　superior　horizontal　isolation　effect　of　the　3D-BIS　than　the　H-BIS　but　greatly　increases　the　horizontal　displacement　and　weakens　the　dynamic　stability　of　the　superstructure.　The　P-　Delta　effect　of　horizontal　elastomeric　bearings　can　usually　be　neglected　for　evaluating　the　displacement　response　of　the　3D-BIS.　Moreover,　providing　additional　rocking　stiffness　is　beneficial　to　decoupling　the　horizontal　translation　from　the　rocking　motion　while　may　increase　the　horizontal　acceleration　of　the　superstructure,　so　proper　rocking　stiffness　should　be　set　for　the　3D-BIS　to　balance　these　effects.　This　study　develops　an　efficient　analysis　means　for　the　seismic　response　of　the　3D-BIS.