Abstract: The maximum principal stress,von Mises equivalent stress and equivalent creep strain of the welded joint of martensitic heat-resistant steel(P91) was simulated by FEM under the condition of 600℃ and 80 MPa.The results show that the maximum principal stress and von Mises equivalent stress are high on the curved points of two sides of groove faces near the fine grain HAZ.The creep strain mainly concentrates in the fine grain HAZ,the maximum creep strain locates in the bottom of fine grain HAZ of the specimen.The stress triaxiality is maximum in the fine grain HAZ and creep cracking occurs because of the intensive constrain of the base metal and weld.The simulation results are in well agreement with the experimental results of crack initiation site and propagation path by using the stress triaxiality as the mechanically controlling parameter of the martensitic heat-resistant steel weld.Therefore,it is reasonable for the stress triaxiality to be used for analyzing the initiation and propagation of type IV cracking in the fine grain HAZ.