In radio astronomy field, the development of terahertz superconducting coherent receivers with ultra-wideband performance is required to achieve the simultaneous observation of multi-spectral lines in much wider frequency band. In response to the application requirements of the next-generation terahertz ultra-wideband receivers, an ultra-wideband ridge waveguide duplexer with a frequency band covering 180-420 GHz (the fractional bandwidth up to 87%) is developed. The main contents include the design and conversion of broadband ridge waveguide filters in 180-300 GHz and 320-420 GHz band, as well as the optimization of terahertz multi-stage coupled ridge waveguide duplexer. The simulation results show that this structure can be in duplex operation in the 180-300 GHz and 320-420 GHz bands with an overall return loss better than 15 dB and the isolation more than 20 dB.