The　terahertz　region　of　the　electromagnetic　spectrum　spans　the　frequency　range　of　0.1THz　similar　to　10THz,　which　means　it　sandwiches　between　the　mid-infrared　(IR)　and　the　millimeter/　microwave.　With　the　development　and　commercialization　of　terahertz　pulsed　spectroscopy　(TPS)　and　terahertz　pulsed　imaging　(TPI)　systems,　terahertz　technologies　have　been　widely　used　in　the　sensing　and　imaging　fields.　It　allows　high　quality　cross-sectional　images　from　within　scattering　media　to　be　obtained　nondestructively.　Characterizing　the　interaction　of　terahertz　radiation　with　multilayer　medium　structures　is　critical　for　the　development　of　nondestructive　testing　technology.　Currently,　there　was　much　experimental　investigation　of　using　TPI　for　the　characterization　of　terahertz　radiation　in　materials　(e.　g.,　pharmaceutical　tablet　coatings),　but　there　were　few　theoretical　researches　on　propagation　of　terahertz　radiation　in　multilayer　medium　structures.　Finite　Difference　Time　Domain　(FDTD)　algorithm　is　a　proven　method　for　electromagnetic　scattering　theory,　which　analyzes　continuous　electromagnetic　problems　by　employing　finite　difference　and　obtains　electromagnetic　field　value　at　the　sampling　point　to　approach　the　actual　continuous　solutions.　In　the　present　work,　we　investigated　the　propagation　of　terahertz　radiation　in　multilayer　medium　structures　based　on　FDTD　method.　The　model　of　multilayer　medium　structures　under　the　THz　frequency　plane　wave　incidence　was　established,　and　the　reflected　radiation　properties　were　recorded　and　analyzed.　The　terahertz　radiation　used　was　broad-band　in　the　frequency　up　to　2　THz.　A　batch　of　single　layer　coated　pharmaceutical　tablets,　whose　coating　thickness　in　the　range　of　40　similar　to　100　mu　m,　was　computed　by　FDTD　method.　We　found　that　the　simulation　results　on　pharmaceutical　tablet　coatings　were　in　good　agreement　with　the　experimental　results　obtained　using　a　commercial　system　(TPI　imaga　2000,　TeraView,　Cambridge,　UK),　demonstrating　its　usefulness　in　simulating　and　analyzing　terahertz　responses　from　a　multilayered　sample.