Using formation netw orking of micro-satellites to observe the earth and miniaturizing microw ave devices has been one of the development trends of space remote sensing. Based on atmospheric microw ave sounder for micro-satellites, tw o high-performance terahertz w aveguide diplexers w ere designed. With mode-matching method, the discontinuity element of diplexer w as analyzed and the parameters of diplexer w ere optimized. In order to improve out-band rejection of 89 GHz filter, different w idths of resonant cavity w as adopted. The simulation results indicated that tw o 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.