By using the Hartmann-Shack wavefront sensor of an adaptive optical system, power spectra density (PSD) of the atmosphere disturbed wavefront overall tilt was measured and analyzed. It is shown that the measured PSDs are different from the theoretic results of Kolmogorov turbulence given by Tyler. A method of calculating the Tyler frequency and the required control bandwidth of the fast steering mirror of an adaptive optical system from measured overall tilt PSDs was discussed too.