Under　seismic　loading,　the　pile　foundations　in　the　liquefiable　soil　are　often　destroyed　due　to　the　liquefaction　of　foundation　soil.　In　this　process,　even　if　the　soil　does　not　reach　full　liquefaction　finally,　the　strength　of　saturated　sand　will　be　weakened　due　to　the　existence　of　the　excess　pore　water　pressure,　which　will　also　lead　to　the　decrease　of　horizontal　resistance　force　of　soil.　If　the　influence　of　excess　pore　water　pressure　on　horizontal　resistance　force　of　soil　is　not　considered,　the　pile　foundations　are　still　designed　by　adopting　the　p-y　curves　in　the　API　standard,　and　the　results　will　be　more　dangerous.　In　this　paper,　the　dynamic　cyclic　torsional　shear　tests　are　carried　out　for　the　Fujian　standard　sands　by　employing　the　vertical-torsional　coupling　shear　apparatus,　and　the　dynamic　characteristics　and　weakened　parameters　of　saturated　sands　in　different　weakened　states　are　studied.　Then　the　formulas　of　ultimate　soil　resistance　are　derived　based　on　the　improved　theoretical　model　of　soil　wedge　at　shallow　layer.　Combined　with　the　theoretical　model　of　flow　failure　around　piles　at　deep　layer,　the　ultimate　soil　resistances　at　different　pore　pressure　ratios　at　any　depth　are　obtained,　and　then　the　p-y　curves　of　pile-soil　interaction　in　saturated　sand　foundation　in　different　weakened　states　are　constructed.　It　can　be　found　from　the　study　that　the　pore　pressure　ratio,　which　characterizes　the　weakened　state　of　soil,　has　a　significant　effect　on　the　ultimate　soil　resistance　in　pile-soil　interaction.　With　the　increase　of　pore　pressure　ratio,　the　weakened　degree　of　soil　will　be　more　serious,　and　the　ultimate　resistance　of　saturated　sand　is　smaller.　That　is　to　say,　the　action　of　the　lateral　loaded　pile　on　the　surrounding　soil　decreases　with　the　increase　of　soil　weakened　degree,　and　the　vice　versa.　©　2020,　Science　Press.　All　right　reserved.