The Joint Total Solar Irradiance Monitor (JTSIM) is due to fly onboard the Fengyun-3E spacecraft and aims to measure the Total Solar Irradiance (TSI) in orbit. The instruments on the Fengyun-3E/JTSIM include the Digital Absolute Radiometer (DARA) from the Physikalisch Meteorologisches Observatorium, Davos and World Radiation Center (PMOD/WRC) and the Solar Irradiance Absolute Radiometer (SIAR) from the Changchun Institute of Optics, Fine Mechanics and Physics Chinese Academy of Sciences (CIOMP/CAS). Radiometers from Switzerland and China will monitor the TSI variability on the same pointing system for eight years. The scientific data from JTSIM will support the analysis of potential long-term trends in the Sun’s variability. In this article, we describe the sensor box and the electronics box of JTSIM, the measurement principle, and the operation mode of SIAR. Before launch, we accomplished some primary calibrations of SIAR in the CIOMP laboratory, including the aperture area, cavity absorption, non-equivalence, diffraction, etc. Other parameters will be calibrated on orbit. The combined uncertainty of SIAR for characterization is 231 – 233 ppm depending on the measurement channel. The characterization of SIAR is an International System of Units (SI)-native scale calibration. An end-to-end calibration against the World Radiometric Reference (WRR) standard or the Total Irradiance Radiometer Facility (TRF) is a procedure where SIAR is directly calibrated with the WRR reference radiometers. The WRR factor for SIAR is 0.99939 – 1.00092 and the combined measurement uncertainty is 0.074% – 0.099%, depending on the measurement channel.