Based on both the theory of optical wave propagation on the slant path and the altitude-dependent model of the ITU-R turbulence structure constant, the modified Rytov method, which is applicable to the optical wave propagation on the horizontal path, is extended to the propagation on the slant path. The uniform model of scintillation index considering inner- and outer-scales is derived and obtained from weak to strong fluctuation regions with Gaussian beam incidence, and can degenerate to the result of the zone outer-scale with atmospheric structure constant. The beam scintillation index of the typical wave band with 1.06 μm,1.315 μm, 3.8 μm and 10.6 μm is analyzed. The numerical conclusions indicate the smaller wavelength, the stronger inner-scale and the inner-scale has a stronger impact on scintillation than outer-scale. But in strong fluctuation, the outer-scale effect is prominence.