Abstract: Based on the theory of optical wave propagation in the slant path and the ITU-R atmospheric refractive index structure constant model which depends on the altitude, and considering the influence of inner scale, we studies the scintillation index from the transmitter to the target and that from the target to the receiver by using the modified Hill spectrum. The relationship of the scintillation index of the echo wave and the Fresnel rate is derived. The influences of inner scale, transmission distance, the beams wavelength and the target height on the echo wave scintillation index are analyzed. The results show that the inner scales and the target heights have more significant effect on the scintillation index than the wavelengths and the transmission distances.