The boundary layer is very important to human activity as the transition zone between the surface and free atmosphere. Turbulence structure of the boundary layer especially the entrainment layer is an important aspect for the boundary layer research, which is very important to improve understanding of the boundary layer and the research of the boundary layer parameterization. Because of the high level of the top of the boundary layer, it is difficult to detect extensively. Generation and evolution of the atmospheric convective layer are simulated in laboratory convective water tank with the dimension of 150 cm×150 cm×60 cm . When the collimated light beam propagates through the simulated atmospheric convective boundary layer, facula image data can be obtained. Using improved covariance method for power spectrum analysis on the facula image, the peak frequency can be found by finding the maximum power spectral density corresponding to the frequency, which is needed to obtain the peak wavelength as the characteristic scale of eddy. It is found that, in the mixed layer, peak wavelength is small, which means that the mixed layer is homogeneously mixed and small scale dominated there. But in entrainment layer the peak wavelength was larger. The average peak wavelength in the entrainment zone has some relation with the convective Richardson number, and the relation are influenced by surface types and convection condition.