Based on the long-term observations from Meridian Project lidars, the density overturning phenomena in the sodium layer above Beijing and Hainan are statistically analyzed. It is found that the density overturning structures generally occur in the bottomside of the sodium layer and typically last for 2?8 h with a width of 2?7 km. The observation results reveal that, the occurrence of those overturning structures are always accompanied with the propagations of large-scale gravity waves, and the disappearance are also related to the breaking process of large-scale gravity waves. These observations indicate that the convective instabilities arising from the interaction of large- and small-scale waves are probably another main reason for the breaking of large-scale gravity wave, except for the dynamic instability and the shear.