Using formation networking of micro-satellites to observe the earth and miniaturizing microwave devices has been one of the development trends of space remote sensing. Based on atmospheric microwave sounder for micro-satellites, two high-performance terahertz waveguide diplexers were designed. With mode-matching method, the discontinuity element of diplexer was analyzed and the parameters of diplexer were optimized. In order to improve out-band rejection of 89 GHz filter, different widths of resonant cavity was adopted. The simulation results indicated that two diplexers had good performance in insertion loss, out-band rejection, return loss, etc. It is proved that mode-matching method is effective in designing terahertz devices.