This paper discussed the principal concept of groundwater reservoir, and presented 4 basic conditions that are demanded by natural water-storing structure of groundwater reservoir such as 1) the reservoir storage condition, 2) the water exchanging condition, 3) the available condition, and 4) the closing condition. This paper also analyzed the influence factors on water-storing structure of groundwater reservoir, indicated the differences between water-storing structure of groundwater reservoir and general water-storing structure, and introduced 3 representative water-storing structures of existing groundwater reservoirs.