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　30m3/(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.