Cavity length has been proven to have a significant effect on the air entrainment of an aerator in flood discharge engineering; however, the estimation of both the bottom and lateral cavity lengths downstream of a sudden fall-expansion aerator remain unclear. This research conducts a series of experiments involving various approach-flow conditions and geometric parameters of an aerator. An improved solution of the cavity length for the bottom and lateral cavities is established. The proposed equation was validated through the data of experiments and predecessor formulas, and exhibited a higher precision than other methods. Both the transverse turbulence and the axial dynamic pressure are found to be related to the formation of a lateral cavity. The present method involving the lateral cavity length was developed based on dimensional analysis and experimental test. The geometric morphology of a lateral cavity exhibits a parabolic shape, which is similar to that of the bottom cavity.