The invention provides a treatment system and method utilizing the biochemically treated tail water of coking wastewater for slag flushing water replenishing, and belongs to the fields of coking workshop coking wastewater treatment and iron making granulated slag production. The treatment system comprises a coking water pipeline, a nozzle, a slag spout, a flushing box and a flushed slag receiving trough, which are sequentially mounted from top to bottom. The treatment system and method are mainly used for iron and steel complexes having coking wastewater treatment demands. Biochemically treated coking wastewater is guided to a granulated slag flushing point of a blast furnace for participating in granulated slag flushing, and residual toxic and harmful substances in the coking wastewater are decomposed by utilizing high-temperature slags. The treatment system and method not only can realize an effect on the non-toxic circulation use of the coking wastewater, but also can partially meet a circulating water replenishing demand in a granulated slag flushing process. The treatment system and method can be used in the new construction or modification of the blast furnace.