The present invention discloses an I-shaped water-retaining dam for an underground reservoir in a coal mine. The I-shaped water-retaining dam is located between coal pillar dams to isolate an underground reservoir from a corresponding coal roadway. The I-shaped water-retaining dam includes an upper flange plate, a web plate, and a lower flange plate from top to bottom, where a vertical face of a dam body is of an I shape; the upper flange plate extends into a roadway roof; two ends of the web plate are embedded into the coal pillar dams; and the lower flange plate extends into a floor. The I-shaped water-retaining dam is located in an underground coal roadway, and bears complex surrounding rock stress. The present invention effectively overcomes water seepage of a weak part at an upper part of a conventional I-shaped water-retaining dam.