Experimental results indicate that an additional lateral deflector built on the outlet can incense the cavity length in aerators with a sudden vertical drop and lateral enlargement. Obsevation shows that the cavity length is affected by both the vertical and Lateral dimensions of the deflector. A larger deflector increases the cavity length substantially and connects the bottom cavity with the sidewall ones which results in more sufficient aeration. However. the backward flow on the bottom may reduce the length of bottom cavity and the effectiveness of aeration. Factors affecting the bottom cavity length, including the slope of the outlet, the bucket angle of sill. and the shape of lateral deflator, should be included in the estimation of the bottom cavity length. Based on test results. an equation has been proposed to predict the cavity length.