The invention discloses construction for transforming a diversion tunnel into an ecological drainage tunnel controlled by a radial gate and a method for the construction. The construction comprises a to-be-transformed diversion tunnel (1); the to-be-transformed diversion tunnel (1) is sequentially divided into a diversion tunnel A section (1.1), an ecological drainage tunnel (1.2) and a diversion tunnel B section (1.3); the tail part of the diversion tunnel A section (1.1) is communicated with the front end of the ecological drainage tunnel (1.2); and the diversion tunnel B section (1.3) is located on the tail end of the to-be-transformed diversion tunnel (1). By adopting the method, the defects in the prior art that a diversion tunnel gate cannot resist a high water head, and a plane gate has a high risk in high water head flowing-water gate closing are overcome. The method has the advantages that continuous streams are maintained during a gate-closing water storage period of the diversion tunnel; water is drained at a large flow; the ecology of a river is maintained; and the water pressure of water blocked by the gate of the diversion tunnel is reduced.