Electromagnetic　theoretical　analysis　was　usually　used　to　investigate　the　pulsed　terahertz　wave　interaction　with　the　medium.　For　epoxy　coatings,　the　material　dispersion　of　the　coating　was　often　simplified　or　ignored　in　the　traditional　propagation　model.　It　is　difficult　to　elaborate　on　the　propagation　mechanism　and　to　differentiate　the　coating　deterioration　as　serving　time　increases.　An　improved　propagation　model　based　on　the　finite-difference　time　domain　was　established　to　characterize　the　propagation　in　the　epoxy　protective　coating　under　broad-band　terahertz　radiation.　Either　an　intact　or　defective　coating　structure　was　investigated,　and　the　simulated　results　were　compared　with　the　experimental　test.　The　dissipation　mechanism　was　considered　in　the　proposed　model.　The　results　indicated　that　the　terahertz　reflections　varied　with　the　change　in　optical　and　dielectric　properties　caused　by　coating　aging,　which　influences　the　intrinsic　impedance　of　the　medium.　Moreover,　it　could　well　elucidate　the　propagation　mechanism　of　pulsed　terahertz　waves　in　rusted,　defective　coating　structures.