Abstract: The corrosion damnification form and corrosion mechanism of some transmission pipeline by transmitting humid gas had been researched by means of field investigating,macro-analysis and micro-analysis with EDS and XRD. The result showed that the corrosion of pipeline inner face was severe uniform corrosion. The corrosion of inside weld where there was no protective coatings was local corrosion. The corrosion mechanism was CO2 corrosion and the corrosion production were FeCO3,FeO(OH) and Fe3O4 mainly.