Abstract: The maximum principal stress, von mises equivalent stress and stress triaxiality in dissimilar welded joints between austenitic(HR3C) and martensitic heat-resistant steel(T91) are simulated by FEM under the conditions of 600℃ and inner pressure of 42.26 MPa. The results show that the maximum principal stress and von mises equivalent stress are quit high in the vicinity of weld/T91 interface, creep cavities are easy to form and expand in the weld/T91 interface. Weld/T91 interface is the weakest part of welded joint. The stress triaxiality peak is located exactly at the weld/T91 interface, and quit high. Therefore, using stress triaxiality to describe creep cavity nucleation and expansion and crack development is reasonable for the dissimilar welded joint between austenitic and martensitic steel.