The invention provides a terahertz duplexer built by using a waveguide filter. The terahertz duplexer comprises a standard waveguide input end (17), a first standard waveguide output end (15), a second standard waveguide output end (16), a high-pass filter (19), a band-pass filter (18) and a T-shaped junction (20), wherein the standard waveguide input end (17) is formed by splicing an upper cavity (13) and a lower cavity (14); the standard waveguide input end (17) is connected with an input end of the T-shaped junction (20); an output end of the T-shaped junction (20) is connected with the high-pass filter (19) and the band-pass filter (18) in two paths; the output ends of the high-pass filter (19) and the band-pass filter (18) are connected with the first standard waveguide output end (15) and the second standard waveguide output end (16) respectively; and the cut-off frequency of the high-pass filter (19) is higher than an operating frequency range of the band-pass filter (18). The terahertz duplexer has the advantages of being small in volume and simple in structure, and is suitable for a microwave radiometer system of a small satellite platform.