The　accurate　extraction　of　the　refractive　index　of　top　coat　(TC)　and　thermally　grown　oxide　(TGO)　layer　in　terahertz　band　is　an　important　condition　for　terahertz　nondestructive　testing　of　thermal　barrier　coatings　(TBCs).　In　view　of　the　fact　that　only　reflection　measurement　can　be　used　for　coating　samples,　we　compare　the　results　of　THz　optical　parameters　and　thickness　extraction　under　the　condition　of　reflection　measurement　and　traditional　transmission　measurement.　Then,　the　refractive　index　of　TC　and　TGO　in　plasma　sprayed　8YSZ　thermal　barrier　coating　(TBCs)　is　extracted　by　using　the　reflection　terahertz　time　domain　pulse　imaging　system,　and　the　thickness　distribution　of　TC　layer　is　further　measured　and　imaged　according　to　the　extracted　refractive　index.　The　results　show　that　the　refractive　index　and　thickness　of　the　sample　can　also　be　accurately　extracted　by　reflection　measurement　in　the　effective　frequency　band　with　small　attenuation.　The　average　refractive　index　of　TC　and　TGO　is　5.23　and　2.91　respectively.　The　refractive　index　of　α-Al2O3,　the　main　component　of　TGO,　is　2.85.　The　average　thickness　of　TC　in　TBCs　is　257　μm.　The　inhomogeneity　of　interface　between　TC　and　bonding　layer　(BC)　can　be　observed　from　THz　image　of　TC　thickness.　Reflection　THz　nondestructive　testing　can　accurately　extract　the　refractive　index　and　thickness　of　TC　and　TGO　in　TBCs,　which　is　of　great　significance　for　the　identification　of　cracks　and　bubbles　in　TBCs　and　the　THz　detection　of　TGO　growth.　©　2021　Journal　of　Mechanical　Engineering.