The　suction　caisson　is　commonly　a　top-closed　cylindrical　steel　structure　with　large　diameter,　short　length　and　much　thinner　skirt　wall　thickness.　The　resistance　to　penetrating　is　calculated　as　the　sum　of　the　tip　bearing　capacity　and　the　adhesion　on　the　both　sides　of　the　skirt　wall.　Since　the　thickness　of　the　skirt　wall　is　very　small,　the　downward　adhesion　produced　by　the　skirt　wall　will　cause　the　additional　vertical　stress　and　shear　stress　in　the　soil　at　the　skirt　tip　level,　increasing　the　skirt　tip　resistance.　However,　the　increase　in　skirt　tip　resistance　caused　by　the　additional　vertical　stress　rather　than　shear　stress　in　soil　at　the　skirt　tip　level　was　only　considered,　this　may　lead　to　an　inaccurate　estimation　for　the　tip　bearing　capacity　and　the　suction　required.　Thus,　a　modified　slip-line　field　is　put　forward　in　this　study　to　estimate　the　tip　resistance.　The　expression　of　obtaining　the　minimum　suction　to　install　the　suction　caisson　in　clay　is　derived　in　terms　of　the　force　equilibrium.　Results　from　calculations　of　the　minimum　suction　have　been　proved　to　be　in　a　good　agreement　with　the　measured　data.