An　effective　approach　to　simulate　the　multi-support　earthquake　underground　motions　is　proposed　in　this　paper　and　the　key　factor　for　this　approach　(i.e.　underground　cross-correlation　function)　is　presented　in　advance　and　elaborated.　Previous　studies　are　mainly　focused　on　the　multi-support　ground　motions　due　to　the　absence　of　the　necessary　conditions　to　simulate　underground　motions,　i.e.,　underground　power　spectral　density　(PSD),　underground　response　spectrum　and,　especially,　underground　cross-correlation　function.　In　this　paper,　the　underground　PSD　and　response　spectrum　are　firstly　derived　and　the　cross-correlation　function　between　the　underground　motions　at　positions　with　different　horizontal　and　vertical　coordinates　is　further　deduced.　The　physical　meanings　of　the　parameters　in　this　approach　are　explicitly　clarified.　Moreover,　a　program　for　generating　the　multi-support　earthquake　underground　motions　is　developed　and　the　reliability　of　the　generated　underground　motions　is　verified.　Finally,　a　two-span　bridge　is　taken　as　an　example　to　investigate　structural　responses　under　multi-support　earthquake　underground　excitations.　Numerical　results　show　that　the　dynamic　responses　under　multi-support　earthquake　underground　motions　are　significantly　different　from　those　under　multi-support　earthquake　ground　motions.　Results　indicate　that　the　simulation　of　multi-support　earthquake　underground　motions　is　significant　for　both　study　and　engineering　application.