For solar irradiance absolute radiometer (SIAR) boarded on meteorological satellites,the main aperture of known area plays an important role.The uncertainty of the main aperture area is at present one of the largest components of the uncertainty in SIAR measurement.The effective area method is a way to measure the main aperture area.It defines the main aperture area based on the aperture of beam- limiting effect in radiometric measurement.Through a superposition of Gaussian beams to generate a known constant irradiance covers the surface of the measured apertures.The ratio of the flux to the irradiance gives the aperture area.We introduce the SIAR,and expound theoretical analysis of the measurement method using Matlab and TracePro software to make simulations.Put forward the specific measurement scheme,and make the uncertainty budget of the measurement device.The combined measurement uncertainty of the main aperture is about 8.2 × 10-5 .This method improves the measurement accuracy of the main aperture,makes the uncertainty of the irradiance measurement about the SIAR from 8 × 10-4 to 6.3 × 10-4 .