Abstract: The causes of early failure of T22/TP347H dissimilar steel welded joints were analyzed by means of chemical composition analysis, mechanical property test, metallographic examination and hardness test. The results show that oxidation notch was found in the side fusion zone of T22 steel. There was an element concentration gradient near the boundary between T22 steel and weld, which produced a martensite embrittlement layer. It was recommended to select welding materials with higher alloy element content or nickel based welding materials to avoid early failure.