Simplified　methods　are　widely　used　in　the　seismic　design　of　underground　structures,　but　they　generally　ignore　the　influence　of　vertical　ground　motion.　This　can　result　in　unreliable　design　because　seismic　earth　pressure　induced　by　vertical　ground　motion　can　have　a　significant　adverse　effect　on　their　seismic　performance.　Therefore,　this　paper　proposes　an　improved　pushover　analysis　method　(I-PAM)　that　applies　distributed　horizontal　and　vertical　inertial　forces　to　simulate　the　mechanical　behavior　of　underground　structures　under　both　horizontal　and　vertical　ground　motions.　The　profile　of　load　distribution　and　target　displacement　is　calculated　from　corresponding　site　response　analysis.　A　rigorous　time-history　dynamic　analysis　method　(TDAM)　based　on　the　finite　element　numerical　analysis　was　employed　in　three　case　studies　to　evaluate　the　performance　of　the　developed　I-PAM.　The　results　showed　that　the　I-PAM　can　accurately　predict　the　structure＇s　peak　axial　force,　bending　moment　and　displacement　under　the　combined　action　of　horizontal　and　vertical　ground　motions,　especially　for　lower　ground　motion　intensity.　It　was　also　found　that　the　responses　of　the　center　column　and　middle　slap　were　calculated　with　higher　accuracy　compared　to　that　of　the　components　in　direct　contact　with　soil　including　sidewalls　and　top　and　bottom　slabs.　The　developed　I-PAM　offers　a　framework　for　further　development　and　enhancement　of　simplified　analysis　methods　for　seismic　design　of　underground　structures.