The present invention discloses a method for constructing inner dump type strip mine pit bottom reservoirs section by section, specifically including the following steps: S1: processing end slopes: discarding clay at a lowest step of an inner waste dump of a strip mine; S2: discharging concrete to slope faces of lowest steps of the end slopes on two sides of a pit bottom; S3: sealing the bottom; S4: discarding gravel into a pit of the strip mine; S5: laying geotextile; S6: re-adopting clay on the lowest steps of the end slopes of the inner waste dump, so as to form a reservoir sealing isolation layer; S7: constructing a plurality of reservoirs step by step in an advancing direction of the strip mine; S8: storing water resources: completing installation of water storage wells; S9: completing installation of water fetching wells; S10: storing water resources.