Abstract: This paper describes a new type of two-stage internal energy dissipators with submerged jet and horizontal swirl flow, aiming at cavitation erosion of the discharge tunnel with internal energy dissipators reconstructed from a diversion tunnel, practical engineering difficulties in increasing effective hydraulic head, and limitation on operation modes and flexibility. Basic flow patterns and hydraulic characteristics are measured and analyzed using scale model tests of tunnel dissipators of this type. The results show that the water level in the shaft is a decisive factor to ensure stable operation, and that the entire flow tends to be stable when this level is above 1.2 times the diameter of submerged-jet tunnel. It is affected by the water levels of the tunnel’s inflow and tail flow, with a more significant effect of the former. In the sections of jet orifice and pipe cyclone orifice, the dimensionless flow rates are increased linearly with the increase in relative net effective heads; in the three sections of submerged jet, horizontal swirl flow and vortex diffusion, wall pressure shows a distinct three-stage drops and a stable variation along each section. Under the experimental conditions in this study, the ratios of pressure head drops in the three sections to the tunnel’s working head are 0.12, 0.47 and 0.2 respectively, and the ratios of net effective heads of the two dissipators, i.e. the submerged-jet section and swirl flow section, are in the ranges of 0.13 to 0.15 and 0.59 to 0.80 respectively. And flow velocity around the two orifices is no higher than 26.2 m/s, and average flow velocities at the outlets of the jet section and vortex diffusion section are 10.5 m/s and 14.5 m/s respectively. All this shows that our design of the tunnel is good and reasonable in its two-stage energy dissipation and basic shapes, but much room is still left to improve the tunnel’s working head and further optimize the allocation of dissipated energy to different stages.