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.