A reflectance transfer spectrometer can be used to accurately and quickly calculate the spectral reflectance of the ground target from the measured spectral irradiances of the sun and ground, thus achieving the long-term stable-radiation standard transfer on-orbit. In this process, a pinhole aperture is used to largely attenuate sunlight for the direct imaging of the solar disk. The accuracy of the pinhole aperture attenuation factor directly determines the measurement and transfer accuracies of the reflectance. Based on the calibration principle of detector response nonlinearity using a laser, a method that combines the wide dynamic standard light source with a back spectrometer for comparison measurements is proposed for the high-accuracy calibration of the attenuation factor of a front pinhole aperture. Results show that the attenuation factor of the pinhole aperture shows obvious nonuniformity both in terms of the spatial and spectral dimensions. After modifying the experimental outdoor reflectance measurements, the relative difference of the measured spectral reflectance of the white diffusing plate can be reduced from 20% to less than 2%. This finding proves the validity of the laboratory calibration of the aperture attenuation factor and can provide a technical basis for the high-accuracy cross calibration of reflectance transfer spectrometers.