The filling water inside the cavity below an aerator occurs for the flow of low Froude number or the small bottom slope of a spillway.The aerator may cease to protect against cavitation damages,and may even act as a generator of cavitation if it is fully filled by water.The experiments were conducted to investigate the influences of the geometric parameters,and then the filling water on the air concentration.The results show that the filling water,or the net cavity length,is closely related to the plunging jet length for a given aerator,and the air concentration at some section is proportional to the ratio /n j L L at a fixed j L for different geometric parameters of aerators.Secondly,at the same ratio of /n j L L,the aerator with a larger height or a larger angle of ramp,or a larger bottom slope,would have a larger plunging jet length,and then a larger net cavity length based on the ratio of /n j L L.As a result,the large space of cavity,or the high air concentration of the flow could be obtained although the filling water increases also based on the fact that=f j n L L L.It is the space of the cavity that is the dominant factor to affect the air concentration of the flow. The filling water inside the cavity below an aerator occurs for the flow of low Froude number or the small bottom slope of a spillway. Aerator may cease to protect against cavitation damages, and may even act as a generator of cavitation if it is fully filled by water the experiments were conducted to investigate the influences of the geometric parameters, and then the filling water on the air concentration. results show that the filling water, or the net cavity length, is closely related to the plunging jet length for a given aerator, and the air concentration at some section is proportional to the ratio / nj LL at a fixed j L for different geometric parameters of aerators. Secondary, at the same ratio of / nj LL, the aerator with a larger height or a angle of ramp, or a larger bottom slope, would have a larger plunge jet length, and then a larger net cavity length based on the ratio of / nj L L. As a result, the large space of cavity, or the high air concentration of the flow could be obtained though the filling water increases also based on the fact that = f j n L L L.It is the space of the cavity that is the dominant factor to affect the air concentration of the flow.