As　underground　structures＇　burial　depth　increases,　buoyancy　resistance　due　to　groundwater　becomes　more　pronounced.　This　study,　through　numerical　simulation,　analysis　of　field　measurement　data,　and　theoretical　analysis,　explores　the　impact　of　changes　in　groundwater　level　on　the　failure　mode　and　uplift　resistance　of　expanded　base　piles　and　proposes　a　new　method　for　calculating　the　ultimate　uplift　capacity　of　expanded　base　piles　considering　the　effect　of　groundwater.　The　research　shows　that　the　rise　in　groundwater　level　significantly　affects　the　uplift　performance　of　expanded　base　piles　by　altering　the　physical　and　mechanical　properties　of　the　soil　and　the　morphology　of　the　pile-soil　failure　surface,　thereby　affecting　the　pile＇s　load-bearing　capacity.　The　study　identifies　a　three-segment　failure　mode　for　expanded　base　piles　and　notes　that　as　the　groundwater　level　rises,　the　extent　of　the　failure　surface　gradually　expands.　Additionally,　the　paper　underscores　the　importance　of　considering　groundwater　levels　in　practical　engineering　design　and　suggests　re-evaluating　the　measured　uplift　capacity　using　the　calculation　method　proposed　in　this　study　to　ensure　engineering　safety.　This　research　provides　a　theoretical　basis　and　computational　tools　for　designing　belled　uplift　piles　under　the　influence　of　groundwater,　offering　significant　reference　value　for　engineering　practice.　©　2024　American　Society　of　Civil　Engineers.