The invention discloses an underground water storage and purification integrated system for a sponge city, which is mainly composed of a grit chamber, a sinking green field, an underground reservoir, and an equipment room. Rainwater flows through grits into the sinking green field for permeation pretreatment, and then flows to the underground reservoir. The underground reservoir is internally provided with a soft rubber dam which divides the underground reservoir into an upper sewage unit and a lower purified water storage unit. The underground reservoir and the equipment room are connected by a pipe. Sewage in the upper part inside the underground reservoir is treated by a treatment system in the equipment room and then flows back to the purified water storage unit of the underground reservoir for storage and reuse. The system integrates functions of rainwater collection, purification, storage and reuse. Rainwater collection, diversion and storage and rainwater treatment and reuse are realized. The system helps to push forward the construction of a sponge city.