A method for constructing a dam inside a dump of an inner-dump strip mine includes: taking an upper surface connection line of a primary water-resisting layer as upper filling reference datum boundary of an artificial water-resisting layer; arranging a dam foundation pit and a trapezoidal abutment on a midline of the dam foundation pit; building and reinforcing a step-shaped retaining dam core wall on the artificial water-resisting layer; laying a foundation impervious layer, waterproof geotextile, and an earth blanket on one side, close to the primary aquifer, of the retaining dam core wall; strengthening advance of a dumping working face on one side, away from the primary aquifer, of the retaining dam core wall, and dumping overburden of a strip mine to form a support; filling a space between the earth blanket and the primary aquifer to form a blocker; and proceeding with construction and forming a continuous retaining dam.