Abstract: The TIG welded joint of TP347HFG/12Cr1MoVG dissimilar steel for platen superheater of a large utility boiler frequently cracked and leaked after serving for 1.5×10⁴ h. The cracking cause of the welded joint between TP347HFG/12Cr1MoVG dissimilar steel was analyzed by means of macroscopic observation, metallographic examination, SEM analysis and mechanical property test. The results show that the early cracking failure of TP347HFG/12Cr1MoVG dissimilar steel joint was mainly caused by the improper welding process, which led to the ineffective fusion of Ni based filler metal and base metal in the fusion zone of 12Cr1MoVG side.