The utility model relates to a device that is used for building and lets out ecological flow under small -size diversion type hydropower station, its characterized in that: near the withered phase main stem of power station damsite upper reaches natural river course concave bank, set up the lower device of letting out ecological flow, the lower device of letting out ecological flow be cylinder suitable for reading open including water intaking prelude and the ecological buret that flows, water intaking prelude, ecological stream buret is in the bottom of cylinder, cylinder upper portion leave the water inlet, slightly higher than riverbed face at the bottom of the water inlet. The utility model discloses under the condition of the building that does not change the diversion formula of building power station, ecological environment protection's requirement has been satisfied promptly in the measure of letting out under the ecological flow of laying, the safety that does not influence the power station electricity generation again and built marine structure. Thereby under can or else influencing the power station electricity generation and having built the condition of marine structure safety, successfully solve a difficult problem of letting out ecological flow down.