In the project of on-orbit calibration, two apertures were designed with nominal diameters of 20 mm and 0.5 mm for switching observations of the earth and the scen, the high precision measurement of the two apertures area ratio is the key to ensure the calibration accuracy. The measurement method of the area of aperture was discussed briefly. Based on the measurements characteristics of the area of aperture, the ratio of the area of the two apertures to the area of the referenced aperture was measured by utilizing laser raster scanning and flux comparison. The area ratio of aperture Φ20 mm to Φ0.5 mm was 1604.381 with a total relative combined standard uncertainty 0.046%. The conclusion provides a feasible solution for the high-precision measurement of the aperture area ratio, and supports the on-orbit calibration of direct solar viewing.