Abstract: About the effect of crack interaction on the pipe welding line,two cracks interaction model was constructed based on finite element method in this paper.Considering the factors of crack dimension and distance,a fitting formula was proposed for stress intensity factors and crack dimension and distances between them.And also an interaction law was discussed.Results show that,with the increase of crack tip distance,the amplification coefficient decreases according to the negative exponential law,tending to 1 finally.And the influence of crack dimension on interaction law is approximately quadric positive correlation.Based on the interaction law and research results,the engineering critical assessment (ECA) for cracks was carried out.Compared with the crack evaluation results with BS 7910 criterion,the proposed fitting formula,which takes interaction effect into consideration,can provide more accurate evaluation results.