Abstract: The expansion and pull-out mechanics processes of the tube-to-tubesheet joints are numerically simulated by using three-dimensional finite element method（FEM） at different expansion pressures.Distributions of the residual contact pressures along axial and hoop directions,and the effect of the expanding pressure on residual contact pressure,pull-out force and wall reduction are obtained.The results show that there are two high residual contact pressure bands（sealing circular-band pressures） near the two surfaces of the tubesheet,and there is a steady residual pressure distribution along the axial direction at central part.The maximum tensile residual stress existed on the inner surface of tube at the transition zone of expansion joints,and which is assumed to be as the main factor for causing the stress corrosion cracks.With increasing the expansion pressure from 240 MPa to 330 MPa,the residual contact pressure,pull-out force and wall reduction increase.The formulas for relating the residual contact pressure and expansion pressure,and the pull-out force and expansion pressure are obtained by analyzing the FEM calculation results.The relationships between the expansion pressure and the joint performance and service reliability are also discussed.