The invention relates to a cast iron air preheater which comprises a shell, a flue gas inlet, a flue gas outlet, an air inlet, an air outlet, heat exchanging elements and a seal structure, wherein the heat exchanging elements and the seal structure are arranged in the shell; the heat exchanging element comprises a plurality of cast iron fin plate heat exchanging units; each cast iron chip plate heat exchanging unit comprises an upper base plate, a lower base plate, inner fins and outer fins which are positioned at two sides of the upper base plate and the lower base plate; and the upper base plate and the lower base plate are aligned longitudinally, and two ends of the upper base plate and the lower base plate are connected through seal bolts, wherein the upper base plates and the lower base plates in the cast iron fin plate heat exchanging units are positioned at the middle secetions among the seal bolts, and the middle sections are uniformly provided with spacing column units with intervals. By the cast iron air preheater provided by the invention, according to the action mechanism of flue gas expanding fin plates, through adopting the technical scheme of adding the spacing column unit, the expanding force pressed in the fin plates is couteracted, and the seal property and heat exchaning effect of the cast iron fin plate heat exchanging units are improved.