In terms of pipeline bridge cracking， the sample was comprehensively detected by spectrum analysis， metallographic microscope， micro hardness tester， scanning electron microscopy (SEM) and energy disperse spectroscopy (EDS). The results showed that the main cause for crack formation included the air holes in welding bead of pipeline bridge， the martensite in heat affected zone， the bainite around welding bead as well as the working environment at 257-310 ℃.