Abstract: Considering the fracture failure of pipeline occurred mainly at the welded seam, the safety of pipeline approximately equal to the safety of the welded seam is put forward, which takes safety probability of welded seam with defects as a random variable, and is subjected to normal distribution. Consider a pipeline with n welded seams, by stratified sampling method from the pipeline with same pipe steel and welding condition, and measure it. A mathematical model is established based on given values, the confidence interval under a given degree of confidence safety probability of the whole pipeline could be obtained.