Light　trapping　has　been　considered　as　an　important　strategy　to　increase　the　conversion　efficiency　of　silicon　thin　film　solar　cell.　It　shows　that　photonic　crystal　with　feature　size　comparable　to　the　wavelength,　for　example,　the　silicon　nanowire　array　has　a　great　potential　to　exceed　the　conventional　Yablonovitch　4n(2)　limit.　Silicon　nanowire　array　has　been　designed　and　constructed　on　silicon　thin　film　solar　cell　due　to　its　excellent　optical　properties.　Generally,　silicon　nanowire　array　is　used　as　the　antireflection　coating,　axial　or　radial　p-n　junction　of　solar　cell.　Different　applications　of　the　silicon　nanowire　arrays　need　different　optical　properties.　Theoretical　investigations　show　that　the　optical　property　is　strongly　dependent　on　the　structural　parameters.　In　this　work,　several　structural　parameters　including　period　(P),　diameter　(D),　height　(H),　and　filling　ratio　(FR)　are　optimized　when　silicon　nanowire　array　plays　different　roles.　Here,　by　using　the　finite　difference　time　domain　(FDTD)　method,　we　focus　on　the　relations　between　the　structural　parameters　and　the　optical　properties　including　reflection　and　absorption　from　300　to　1100　nm.　In　the　FDTD　simulation　model,　the　substrate　material　is　crystal　silicon　film,　and　the　silicon　nanowire　array　is　on　the　surface　of　the　substrate.　In　this　calculation,　the　top　and　the　bottom　of　the　unit　cell　are　air　with　perfectly　matched　layers,　and　with　periodic　boundary　conditions　at　the　side　walls.　When　the　silicon　nanowire　array　is　used　as　the　antireflection　coating,　the　silicon　nanowire　array　shows　a　lowest　reflection　(7.9%)　with　H　=　1.5　mu　m,　P　=　300　nm,　and　FR　=　0.282.　When　silicon　nanowire　array　acts　as　axial　p-n　junction　solar　cell　(the　p-n　junction　is　formed　by　substrate　and　nanowire　array),　the　absorption　efficiency　reaches　a　maximum　value　of　22.3%　with　H　=　1.5　mu　m,　P　=　500　nm,　and　FR　=　0.55.　When　the　silicon　nanowire　array　acts　as　the　radial　p-n　junction　solar　cell,　the　absorption　efficiency　could　obtain　a　maximum　value　of　32.4%　with　H　=　6　mu　m,　P　=　300　nm,　FR　=　0.349.　In　addition,　the　optical　properties　of　silicon　nanowire　array　with　random　diameter　and　position　are　also　analyzed　here.　The　absorption　efficiency　of　optimized　random　silicon　nanowire　array　reaches　27.8%　compared　with　a　value　of　19.9%　from　ordered　silicon　nanowire　array.　All　of　these　results　presented　here　can　provide　a　theoretical　support　for　the　silicon　thin　film　solar　cell　to　increase　the　efficiency　in　the　future　application.