The invention relates to method and system for treating water in the field of ironmaking, in particular to method and system for treating blast-furnace flushing slag water. The system for treating blast-furnace flushing slag water comprises a horizontal sedimentation tank, a slag filtering tank, a flushing slag backwater channel, a filtering cage and a water collecting groove, wherein a self-sucking pump and a submersible sewage pump are arranged in the flushing slag backwater channel, the self-sucking pump and the submersible sewage pump are connected with the filtering cage by a steel pipe, an upper overflow water channel of the filtering cage is connected with the water collecting groove by the steel pipe, the water collecting groove is connected with the slag filtering tank by the steel pipe, and a lower water outlet channel of the filtering cage is connected with the horizontal sedimentation tank by the steel pipe. In the invention, the flushing slag water is filled into the filtering cage by arranging the self-sucking pump and the submersible sewage pump in the flushing slag backwater channel, the upper part and the lower part of the filtering cage can discharge water, thick slag water which flows from the lower part can enter the horizontal sedimentation tank, filtered water which flows from the upper part enters the slag filtering tank through the water collecting groove, and the flushing slag backwater does not enter the original treating system, thereby solving the problem of discharging the flushing slag water outwards and maintaining the normal operation of production.