Abstract: The fatigue crack propagation mechanism of spiral-weld pipeline was analyzed. The crack propagation rate of base metal and weld metal were tested. In order to further understand the reason why weld metal and base metal have different crack propagation rate, scanning electron microscope was used. The results showed that base metal had a lower rate of crack propagation than that of weld metal. Many deep dimples and surface cracks were distributed in weld metal, led to the organization loose and reduced the toughness of weld metal, finally reduced the durability of pipeline. Spiral weld was susceptible to suffer from fatigue fracture under circulating fluctuation loading.