A step spillway placed behind X-shaped flaring gate pier was proposed and applied in practical engineering. If the first step is higher than the others, cavity will exist between the nappe and the step spillway.The flow with air entrainment will occur on the flow surface of steps. Under this condition, the step can be protected from damage at the discharge over 30 m3/(ｓ·m). The relation of the cavity size, the weir head, the difference between the weir crest and the exit of the flaring gate pier, the drop height, the weir end slope and the step slope were studied in this paper. The formula was deduced to calculate the length of cavity. Calculating results were identical with that of numerical stimulations and experiments. It is shown that the cavity is mainly affected by the drop height, the weir end slope and the step slope.
