The invention discloses a nanometer fluid for a blast furnace cooling system and a preparation method thereof. The fluid comprises soft water or a desalted hydrosolvent, a nanometer oxide solute of Al2O3 or CuO, wherein, the pH value of the soft water or the desalted hydrosolvent is equal to 8.5 to 9.5, the granularity of the nanometer oxide of the Al2O3 or the CuO is 50nm to 100nm which accountsfor 0.5 percent to 1.0 percent of the volume percentage of the solvent. The preparation method of the nanometer fluid includes: adding a pH regulating agent with the volume percentage of 2.0 percent to 2.3 percent into the soft water or the desalted hydrosolvent, adjusting the pH value of the soft water and simultaneosly increasing the whole water pump pressure of an enclosed cooling system of a blast furnace to form a stable suspension, thereby improving heat transmission efficiency, reducing the use amount of cooling water by 20 percent to 25 percent, decreasing the consumption of the cooling water, and solving the problem that the existing preparation method of the nanometer fluid can not be applied to the large blast furnace.