Abstract: By means of macroscopic examination, geometric measurement, OM analysis, fracture morphology analysis, Vickers hardness test, the cracked F91/TP310HNbN welded joints of last stage superheater of USC boiler in a power plant was investigated.The results show that the chemical composition, microstructure, mechanical properties and physical properties of the weld have no big difference from those of the stainless steel base metal but have big difference from those of the F91 base metal, as a result, the F91 side weld fusion line becomes the weak link of the whole joint, under a variety of stress concentration, the cracking of the welded joint occurs. In order to prevent the early failure of dissimilar steel welded joints, more suitable welding materials should be selected, and metal supervision and inspection should be strengthened when the unit is shut down.