The　traditional　construction　methods　of　terahertz　demultiplexers　and　grating　couplers　need　to　be　calculated　with　the　help　of　classical　theory　and　experience,　so　their　design　flow　is　complex,　and　their　performance　depends　on　the　unit　structure　parameters.　With　the　proposal　and　application　of　reverse　design　method,　the　device　structure　that　meets　the　functional　requirements　can　be　designed　on　a　limited　size　substrate.　Based　on　this,　the　reverse　design　method　is　applied　to　the　design　of　terahertz　demultiplexer　and　grating　coupler　with　dimensions　of　3　mm×3　mm×200　μm　and　12　mm×12　mm×200　μm,　respectively.　The　simulation　results　of　finite-difference　time-Ddomain　(FDTD)　show　that　the　terahertz　demultiplexer　can　perfectly　separate　a　beam　of　terahertz　waves　with　two　frequencies　from　two　ports,　and　the　transmittance　is　more　than　0.75　at　0.500　THz　and　0.417　THz,　and　the　crosstalk　between　adjacent　channels　is　less　than　-19　dB.　The　coupling　efficiency　of　the　terahertz　grating　coupler　is　as　high　as　0.85　at　0.32　THz.　©　2022,　Chinese　Lasers　Press.　All　right　reserved.