Based on the temporal covariance of logarithmic amplitude and the practical conditions, the analytical expression is obtained for aperture receiving power spectrum of atmospheric scintillation of satellite-to-ground downlink. The effect of the receiving aperture and zenith angle on the power spectrum of atmospheric scintillation is analyzed. The experiment of detecting stellar intensity is accomplished with a large antenna. The real power spectrum of atmospheric scintillation is obtained. The results of experiment accord well with these of theoretical analysis. The results show that the power spectrum of atmospheric scintillation includes low-frequency and high-frequency spectrum. The amplitude of low-frequency spectrum is constant. The high-frequency spectrum obeys the power law and the value of power-law is -11/3. The amplitude of power spectrum gradually reduces with the aperture scale adding or the zenith decreasing, and the shape of power spectrum is unvariable.