To　realize　the　reduction　of　calculation　time　and　memory　usage　for　seismic　wave　modeling　on　large　scale　model,　we　propose　a　3D　trapezoid-grid　finite　difference　time　domain　(FDTD)　method.　It　adopts　the　size-increasing　trapezoid-grid　mesh　to　fits　the　increasing　trend　in　depth　of　seismic　wave　velocity,　which　can　significantly　reduce　the　oversampling　in　high-velocity　region.　The　trapezoid　transformation　is　used　to　alleviate　the　difficulty　of　processing　irregular　grids.　We　derive　the　3D　acoustic　equation　with　the　Convolutional　Perfectly　Matched　Layer　(CPML)　absorbing　boundary　condition　in　the　trapezoid　coordinate　system.　Numerical　tests　are　given　to　verify　the　effectiveness　of　our　method.　With　comparable　accuracy,　our　method　can　achieve　about　54.1%　improvement　on　efficiency　and　88.7%　reduction　on　memory　compared　with　regular-grid　FDTD　method.　©　2021　Society　of　Exploration　Geophysicists　First　International　Meeting　for　Applied　Geoscience　&　Energy