Abstract: Cracking leakage occurred to the T91/HR3C dissimilar steel welded joint used on high temperature reheater of an ultra-supercritical concurrent boiler after 168 h of service. The cracking causes of the welded joint were analyzed by means of macroscopic examination, chemical composition analysis, hardness test, metallographic examination, fracture analysis and so on. The results show that the improper welding process led to the coarsening and excessively large residual stress in the heat affected zone (HAZ) of T91 steel pipe side, and type IV cracks produced in the HAZ under the effect of excessive residual stress, which resulted in the cracking leakage of the welded joint. Corresponding suggestions were put forward at last.