Based on microsatellite constellation, meteorological forecast has been greatly improved in terms of temporal and spatial resolution. To separate frequency in RF front-end system of the microsatellite atmospheric microwave sounder, a waveguide diplexer with compact size and low insertion loss is adopted. A 166/183GHz diplexer is designed using equivalent circuit method and mode-matching technology. According to the actual situation of machining, the effects of iris steepness and iris thickness on the performance of diplexer are analyzed in simulation. Through the analysis of the split-block realization and the machining defects, the way of machining is continuously optimized to obtain a satisfactory machining sample. Maximum measured insertion loss is 1.5dB, and minimum measured return loss is 15dB. Measured out-band rejection is higher than 27dB. Good agreement between simulation and measurement has validated the feasibility of designing method.