Abstract: The simulation of multi-layer and multi-layer welding process was realized based on the layer-by-layer activation modeling method,and the mixed heat source model was used to establish the finite element model of multi-pass and multi-layer welding for thick X70 steel plates with different thickness. The evolutions of welding temperature field and stress field were simulated and analyzed. The non-fusion defects and equivalent pipeline working conditions were introduced into the model. The influence of non-fusion defects on post-welding residual stress distribution and X70 pipeline safety was simulated and analyzed. The results indicated that, the maximum Mises stress at the repair welding seam containing non-fusion defects was 470 MPa, which was below the yield strength of base steel of X70 pipeline steel. The highest Mises stress under equivalent working conditions was located in the non-fusion defects and reached 592 MPa, which exceeded the yield strength of the parent metal but did not reach its tensile strength. The simulated results was in good agreement with the experimental results, which indicated that the founded model had high reliability and accuracy.