Entrainment zone (EZ) at the top of convective boundary layer is a transition layer between the atmospheric convective boundary layer (ACBL) and free atmosphere, the turbulence structure of entrainment zone is one of the main properties. Because of the high level of entrainment zone, it is difficult to detect extensively, and little comprehension is obtained. Generation and evolution of the atmospheric convective boundary layer are simulated in laboratory convective water tank with the dimension of 150 cm×150 cm×60 cm. When the collision light beam propagates through the simulated atmospheric convective boundary layer, distribution of intensity of beam is fluctuating. Based on the theory of light wave propagation in turbulent media, the characteristic of temperature structure constant in entrainment zone can be given. In entrainment zone, the normalized temperature structure constant accords with rule. At the level 0.8zi, the temperature structure constant is minimum, then increases gradually with height, and gets maximum at the level zi. Over the level zi, the temperature structure decreases gradually with height, and gets to zero at the level 1.2zi. At the level zi of the boundary top, the normalized temperature structure constant varies within a certain range of 0.08～0.3, this could be contributed to the large-scale structure in entrainment zone.