Abstract: The energy dissipator of flaring gate pier, stepped spillway and stilling basin has effectively solved the problems of high-velocity flows that have occurred under large discharge per unit width and high working head of the dams in China. It often adopts a transition ladder connecting the flaring gate pier to stepped spillway section. This paper presents a numerical simulation study on the transition ladder design for the spillway of Ahai hydropower station, using a 3-D flow model equipped with a RNG turbulence model, water-gas two-phase VOF method, PISO algorithm for velocity-pressure coupling, and a geometry reconstruction scheme for unsteady flow iteration solution to generate free surfaces. The 3-D flow simulations were conducted for four design schemes of this integrated energy dissipator: three schemes with transition ladders of one step, two steps and three steps respectively and the same step sizes of 2 m high and 1.5 m wide, and one scheme for the primary design with no transition ladder. Simulations show that in all the ladder schemes, negative pressure occurs in the head ladder and pressure on the solid surface of the head ladder is distributed in the same pattern. And in all the three cases, the lowest pressure on vertical step faces is negative and occurs at Section 22.56 m, with its value (-0.90 kPa) significantly higher than that of the primary design case (-4.47 kPa). On the horizontal step faces of the transition ladder cases, the lowest pressure (-0.60 kPa) is also higher than the corresponding value (-3.90 kPa) of the primary design. This indicates that the number of ladder steps has no considerable effects on the negative pressure or its distribution. Its effect on energy dissipation, however, is significant. Our results reveal that of the three ladder schemes, the two-step transition ladder can achieve the highest energy dissipation ratio (34.2%) and its head step has a much greater dissipation effect than the other one.