Abstract: The dielectric constant and electical conductivity of underground pipeline are obviously different with surrounding medium,regardless of its internal filling air,water or other medium.It is prerequisite with the ground penetrating radar for detecting underground pipelinse.This paper analyzes the ground penetrating radar for detecting underground pipelines polarity.Through the establishment of metal and non-metallic pipeline forward numerical model, it focuses the transimission mechanism of GPR electromagnetic wave in metal pipeline,nonmetal pipeline. Combined with the actual detection of underground pipeline ground-penetrating radar feature mapping,it analyzes the spectrum characteristics from the reflection waveform,polarity,phase axis,electromagnetic wave energy absorption and attenuation.Finally,it establishes the identificaion method of GPR detecting metal and non-metallic pipeline.The results play a very good guidance for accurately detecting underground pipelines.