Two phases of radiation reception and electrical calibration are needed for absolute radiometer to measure radiation. Because absolute radiometer is thermoelectricity detector, at the two phase temperature response of the cavity could be compared after the receive cavity of the absolute radiometer reaches balance, and then the radiation value is electrically calibrated. According to the law that the temperature of the reception cavity rises up exponentially when radiated or heated up, dynamic forecast of the temperature is adopted and then it gets compensation by heating up with electricity. A new method of measurement is worked out under the state that the temperature of the cavity maintains invariable both at the phases of observation and calibration. The rapid measurement method was used on two Solar Irradiance Absolute Radiometers (SIARs) named SIAR-2a and SIAR-2b constructed for Physikalisch-Meteorologisches Observatorium Davos/World Radiometric Center (PMOD/WRC). Comparison experiments were carried at WRC for a year and a half. The sun was observed together with the instruments of WSG more than 10000 times at the same time. The data obtained from PMOD/WRC shows that its absolute precision reaches 0.08%. The precision of the new method is same with the old one (the balance is slower and the measurement time is longer) but the measurement cycle is shorter.