Based on the smooth perturbation theory, a general expression of normalized spectrum for Gaussian beam and an analytic formula of normalized speetrum for collimated or spherical beam in a homogeneous path are given. Caleulation shows that there is a peak frequency in these spectrums and the frequency depends on turbulent outer scale and transversal wind. According to this eharaeteristie the outer scale is remote sensed successfully by using the atmospheric eoherent length measurer. At the same time five three-cup anemometers distributed evenly on the path are applied to measure average wind. Experimental results indicate that the outer scale is varied in the range of 2.6～7.0 m at the altitude of 8 m above the ground and the average value is 4.9 m.