In 2019，the FORUM (Far-infrared Outgoing Radiation Understanding and Monitoring) mission has been selected by the ESA as the ninth mission Earth Explorer in the Living Planet Programme. It will measure the emission spectrum of the Earth′s stratosphere and troposphere in the spectral range of 100 to 1600 cm-1(i.e.6.25 m to 100 m). The FORUM space segment consists of a single satellite carrying two optical instruments: the FORUM Sounding Instrument (FSI) and the FORUM Embedded Imager (FEI). The primary optical instrument is an infrared-atmospheric-sounder Fourier transform spectrometer. During the FORUM phase A/B1, studies have been led by Airbus Defence and Space (UK) and Thales Alenia Space Ltd (UK) as mission prime contractors, with concept A and concept B respectively. This paper will describe the design and key technology research progress of concept B of the FSI and FEI.