The invention provides a method for reconstructing a temporary diversion tunnel into a permanent
flood discharge facility. The method comprises the steps that firstly, an entrance of the original diversion tunnel (1) is blocked; secondly, an entrance curve section of the diversion tunnel and a straight section behind the entrance curve section are reconstructed into an energy dissipation pressure forebay (2), and the wall surface of the
tail end of the energy dissipation pressure forebay is provided with a
jet flow outlet (3), so that the diversion tunnel section behind the outlet becomes a
free flow section (6) of a permanent
flood discharge tunnel, wherein the straight section is at least five times longer than the entrance curve section; thirdly, at least one
flood discharge tunnel (4) is newly constructed at the position above the entrance of the blocked diversion tunnel, wherein the bottom surfaces of entrances of the newly constructed flood
discharge tunnels are at least 2m higher than the highest position which
silt silting at the entrances can reach; and fourthly, each newly constructed flood
discharge tunnel (4) is connected with the energy dissipation pressure forebay (2) through at least one connecting pipeline (5), and the axes of the newly constructed flood
discharge tunnels (4) intersect with the axis of the energy dissipation pressure forebay (2). By means of the method, not only can the diversion tunnel with the
planar curve section be smoothly reconstructed and utilized, and investment is saved for a whole water conservancy and water power project, but also the energy dissipation effect can be improved, and the service life of a building is prolonged.