A　series　of　model　tests　were　conducted　on　Perspex-made　suction　caissons　in　saturated　dense　marine　sand　to　study　the　sand　plug　formation　during　extraction.　Suction　caissons　were　extracted　by　pullout　loading　or　by　pumping　air　into　the　suction　caisson.　Effects　of　the　pullout　rates,　aspect　ratios　and　loading　ways　(monotonic　or　sustained)　on　the　pullout　capacity,　and　plug　formation　were　investigated.　It　was　found　that　the　ultimate　pullout　capacity　of　the　suction　caisson　increases　with　increasing　the　pullout　rate.　The　sand　plug　formation　under　the　pullout　loading　is　significantly　influenced　by　the　pullout　rate　and　the　loading　way.　When　the　suction　caisson　is　extracted　at　a　relatively　slow　rate,　the　general　sand　boiling　through　the　sand　plug　along　the　inner　caisson　wall　occurs.　On　the　contrary,　the　local　sand　boiling　will　occur　at　the　bottom　of　the　suction　caisson　subjected　to　a　rapid　monotonic　loading　or　a　sustained　loading.　Test　results　of　the　suction　caisson　extracted　by　pumping　air　into　the　caisson　show　that　the　pressure　in　the　suction　caisson　almost　follows　a　linear　relationship　with　the　upward　displacement.　The　maximum　pressures　for　suction　caissons　with　aspect　ratios　of　1.0　and　2.0　during　extraction　by　pumping　air　into　the　caisson　are　1.70　and　2.27　times　the　maximum　suction　required　to　penetrate　the　suction　caisson　into　sand.　It　was　found　that　the　sand　plug　moves　downward　during　extraction　by　pumping　air　into　the　caisson　and　the　variation　in　the　sand　plug　height　is　mainly　caused　by　the　outflow　of　the　sand　particles　from　the　inside　of　the　suction　caisson　to　the　outside.　When　the　suction　caisson　model　is　extracted　under　the　pullout　rate　of　2mm/s　(0.28mm/s　for　the　prototype),　the　hydraulic　gradient　along　the　suction　caisson　wall　increases　to　the　maximum　value　with　increasing　the　penetration　depth　and　then　reduces　to　zero.　On　the　contrary,　when　extracted　under　the　pullout　rate　of　10mm/s　(1.4mm/s　for　the　prototype),　the　hydraulic　gradient　along　the　suction　caisson　wall　increases　with　increasing　the　pullout　displacement.　When　extracted　by　pumping　air　into　the　caisson,　the　hydraulic　gradient　reaches　the　critical　value,　and　at　the　same　time,　the　seepage　failure　occurs　around　the　suction　caisson　tip.