Abstract: The stepped spillway was used to design the Xinshe old inverted siphon of Yan'an Yellow River diversion work.The improvement RNG k-ε turbulence model and SIMPLEC algorithm were used, with 3D numerical simulation, it researched its hydraulic features.With the 1:1.2 slope ratio, it analyzed the change rule of time pressure distribution,velocity distribution and air ration of design working condition with the different step sizes stepped energy dissipater.