Abstract: Dissimilar steel welding seam of the secondary platen superheater of a supercritical unit cracked and leaked. The cracking reasons of the dissimilar steel welding seam were analyzed by using macroscopic observation, metallographic examination, hardness test, scanning electron microscope analysis and energy spectrum analysis and so on. The results show that the disharmony between the expansion and deformation of the two platen superheater on the same screen leads to the low-cycle fatigue cracking of the dissimilar steel welding seam. On the basis of the analysis of the causes of the cracking of the welding seam, the method of measuring the stress concentration level of the metal magnetic memory was put forward, which can effectively prevent the crack of the dissimilar steel welding seam.