n order to studying long-term climate variability and globe environment change, Solar Total Irradiance Monitor (STIM) is constructed planed to aboard on FY-3 sun-synchronous polar orbit weather satellites. STIM is made up of three same absolute radiometers, which are similar with Solar Irradiance Absolute Radiometers (SIAR). We designate one radiometer as revising radiometer. It works only one day once one or two months to demarcate the drift of the other two radiometers. Solar irradiance is measured when the sun scans over the field of view of the absolute radiometers, respectively. The irradiance measurement is carried on at the position near the North Polar after the satellite move out the shade of the earth. SIAR is electro-calibrated cavity absolute radiometer. SIAR-1 had completed The Ninth International Pyrheliometer Comparisons (IPC-IX) with World Radiometric Reference (WRR) in Physikalisch-Meteorologisches Observatorium Davos /World Radiation Center (PMOD/WRC), Switzerland from 25 September to 13 October 2000, the comparison results showed that SIAR-1 is 0.078% higher than WRR. So SIAR-1 was given a WRR calibration factor 0.99922. Where after, we compared STIM with SIAR-1, and draw a conclusion: that the absolute accuracy of STIM is better than 0.2%.