In atrocious working conditions, corrosion is one of the major failure modes for underground pipelines. A method for monitoring pipeline corrosion applied fiber Bragg grating(FBG) sensor is proposed. Internal high pressure of pipes caused by carrying oil, gas and other matters will expand the pipe and then result in hoop strain on pipe outer wall. Assuming the pipeline has an indefinitely length, the axial strain pipe outer wall caused by internal pipe pressure is negligible. The pipe wall will be thinning at corrosion, while the pressure inside the pipe remains about the same, and this will increase hoop strain of the outer wall. Pipe wall and the strain are to some extent in inverse proportion when internal pressure keeps constant. This method attached packaged fiber grating strain sensors to the surface of pipe, the corrosion extent of pipe can be judged by monitor hoop strain. Through detailed contrastive analysis of theoretical value, analogue value and test value to explain the method is practical and worth being extensively applied and spread, and then provide strategy of disaster prevention and reduction for underground pipeline.