Abstract: A wide range of cracking was found on the partition plates of rotary air preheaters in a 300 MW unit, and the cracking was mainly located in the toe area of fillet welds between the radial partition plates and the central cylinders, and also between the radial partition plates and circumferential partition plates. The reasons for the cracking were analyzed from the aspects of physical and chemical properties, structure and external factors. The results show that the cracking of the preheater partition plates was mainly caused by the thermal cracks and welding defects due to the improper welding technology parameters.