In this study, an on-board calibration hot load (CHL) unit was developed for a terahertz sensing system (THzSens) receiver at 480 GHz to observe the emission lines of water and oxygen in the atmosphere. The compact CHL unit can be mounted on a microsatellite. We propose an accurate calibration method and a unit compact in design. We conducted experiments in order to obtain the best possible return loss value below the required −60 dB for calibration. The CHL unit uses two specular reflection absorber (SRA) plates and a radar absorbing material (RAM) as the absorber termination. The absorbers are assembled in a lightweight case made of carbon-fiber-reinforced plastics. The SRAs are placed at the Brewster angle with respect to the incident radiation to minimize reflection, which was found experimentally at 480 GHz. The CHL design is expected to have less than −78 dB reflection loss including SRA back scattering at 480 GHz. The return loss as a function of frequency for this design is below −60 dB, meeting the design requirements. Further, a compact prototype CHL was manufactured based on the proposed quasi-optical design. The new CHL for 480 GHz is 1/10 in the volume of the previous SMILES CHL for 650 GHz. The manufactured CHL prototype will be evaluated as a calibrator using the transmitter/receiver setup used in this study.