Continuous integrated simulation of JCO forming, reducing gap, welding, and mechanical expanding processes of large-sized straight welded pipe is conducted using Marc software. Taking expanding ratio as variable, the residual stress distribution of weld bead and the influence rule of the expanding ratio are investigated before and after expansion. In thickness direction, in the bonding zone of pre-welding and outside weld, as well as at the bottom of the inside weld, the residual stress is large before expanding. The residual stress in the outside weld is large after expanding. The peak value of residual stress is decreased by about 61 to 78%. The expanding ratio is higher, the peak clipping effect becomes more evident, and residual stress becomes more uniform. The simulation results agrees with the test results, thus confirm that integrated simulation can achieve the visualization of residual stress evolution in the production process of large-sized straight welded pipe.