This　paper　focuses　on　the　study　of　simulations　for　spatially　variable　seismic　underground　motions　in　U-shaped　canyons.　First,　a　canyon　ground　cross-coherence　function　based　on　commonly　used　coherence　function　models　of　flat　terrain,　is　deduced　and　presented.　To　further　obtain　the　underground　cross-coherence　function,　a　mathematical　expression,　including　its　specific　deduction　process　for　describing　the　relationship　between　coherence　functions　of　multi-support　ground　and　underground　motions,　is　also　given　in　detail　and　adopted.　Then,　the　key　factors　(i.e.　canyon　underground　power　spectrum　density　and　canyon　underground　coherence　function)　for　simulating　the　spatially　variable　seismic　underground　motions　are　obtained　by　a　two-step　transferring　method　from　flat-ground　to　underground　soil.　Furthermore,　a　program　is　coded　for　generating　the　spatially　variable　seismic　underground　motions.　The　effectiveness　of　the　generated　seismic　motions　is　further　verified.　Finally,　two　numerical　examples　are　taken　to　validate　the　proposed　simulation　approach,　illustrating　the　specific　characteristics　of　canyon　coherence　function.　The　analysis　results　show　the　apparent　differences　of　the　simulated　seismic　motions　using　the　canyon　coherence　function　from　those　using　conventional　coherence　function　models　of　flat　terrain.　The　proposed　approach　provides　some　insights　for　anti-earthquake　analysis　of　soil-structure　interaction　or　underground　structures　in　canyon　topography.