The　reverse　design　method　(RDM)　is　a　frontier　direction　in　the　optical　research　field.　In　this　work,　RDM　is　applied　to　the　design　of　terahertz　demultiplexers,　including　two-port　and　three-port　terahertz　demultiplexers,　with　areas　of　3　mm　×　3　mm　and　5　mm　×　5　mm,　respectively.　The　Finite-Difference　Time-Domain　(FDTD)　simulation　results　show　that　the　terahertz　waves　at　frequencies　of　0.5　THz　and　0.417　THz　can　be　well　separated　by　the　two-port　demultiplexer,　and　the　transmittances　of　the　two　outputs　reach　as　high　as　0.75　after　bandwidth　optimization.　Meanwhile,　the　three-port　terahertz　demultiplexer　can　have　terahertz　waves　separated　from　three　Ports,　and　the　crosstalk　between　adjacent　channels　is　less　than　−18　dB.　©　2021　by　the　authors.　Licensee　MDPI,　Basel,　Switzerland.