An acid replenishing system includes an acid tank, a draining apparatus, an acid replenishing apparatus, and a control unit. The acid tank contains a used acid solution. The draining apparatus drains an amount of the used acid solution from the acid tank. The acid replenishing apparatus replenishes an amount of a replenishing acid into the acid tank. The control unit controls the draining apparatus and the acid replenishing apparatus to perform a plurality of acid replenishing stages, so a total set amount of the replenishing acid to be added into the acid tank has been replenished.