An opencast coal mine underground water reservoir comprising an impermeable layer and, provided below the impermeable layer, a water storage space and a purification layer. The water storage space comprises a first water storage space and a second water storage space. The purification layer comprises a first purification layer and a second purification layer. The first purification layer is provided horizontally in the water storage space and divides the water storage space into the first water storage space and the second water storage space. The first water storage space is provided below the impermeable layer and between same and the first purification layer. The second water storage space is provided below the first water storage space and the bottom of the second water storage space is provided at the bottom of the opencast coal mine underground water reservoir. The second purification layer is provided vertically within the second water storage space. The present invention, by constructing the opencast coal mine underground water reservoir, prevents wastage of water resource produced during a coal mining process, and by providing the purification layer in the opencast coal mining underground water reservoir, implements repeated purification of water, prevents environmental pollution, and implements a water conservation mining policy during the coal mining process.