A　non-uniform　segmented　grating　coupler　with　a　metal　reflective　layer　is　designed　by　finite　difference　time　domain　simulation.　The　grating　coupler　is　on　a　400　nm　thick　z-cut　lithium　niobate　thin　film　waveguide.　A　metal　reflective　layer　is　added　to　reduce　the　leakage　to　the　substrate,　and　a　non-uniform　grating　structure　is　constructed　to　improve　the　mode　mismatch.　The　non-uniform　grating　coupler　consists　of　three　segmented　grating　regions,　with　a　gap　between　two　adjacent　grating　regions.　The　grating　periods　of　these　three　grating　regions　are　not　completely　the　same,　namely　?1,　?2,　and　?3,　?1　is　shorter　than　?2　and　?3.　According　to　grating　coupling　principle,　the　key　structural　parameters　of　the　grating　coupler　such　as　the　gap　between　the　grating　regions,　the　grating　period,　the　duty　cycle　are　discussed　with　respect　to　the　grating　coupling　efficiency.　Through　FDTD　simulation　calculation,　the　coupling　efficiency　under　theoretical　simulation　(TM　mode,　at　1550　nm)　can　reach　nearly　76%.　The　proposed　non-uniform　segmented　grating　coupler　is　expected　to　be　fabricated　and　applied　to　the　input-output　coupling　of　waveguide　devices.　©　2022　SPIE