It is necessary to design an appropriate system bandwidth for an adaptive optical system to correct the atmospheric turbalence. Because the theoretical estimation for the turbulence is more different from the real atmospheric tarbulence,this paper researched the method to measure the bandwidth accurately.Through measurement of the wavefront passed through 500 m of horizontal turbulence, the phase power spectral density of turbulence wavefront was analyzed, from which the circadian variation of the bandwidth requirement was obtained for first time. It shows that the bandwidth requirement is nearly steady around 10 to 15 Hz in night time, and strongly fluctuated around 20 to 90 Hz in day time. Furthermore, it points out that the total sampling time for the estimated bandwidth must be greater than 70 s in order to acquire a more precise result. There experimental results can provide useful data for designing and can fabricate an more reasonable adaptive optical system.