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The singularity expansion method (SEM) assumes that the electromagnetic scattering wave of an object has a fixed poles distribution, i.e. the resonant frequency is fixed. So the object can be identified by extracting poles from radar data. While studying ground penetrating radar (GPR) echo signal features of nonmetallic pipelines, the paper finds that the GPR signal's frequency center downshifts when a nonmetallic pipeline is placed in an underground half space, when compared with it being placed in free space. The FDTD calculation model for a nonmetallic pipeline placed in different spaces is established. Through model calculation, the result shows the same phenomenon as the test data. More calculations indicate that the phenomenon of GPR echo frequency center shift has little relationship with the depth of the nonmetallic pipeline underground, but relates closely to the relative permittivity of the nonmetallic pipeline and the surrounding medium. Furthermore, from the phase change when electromagnetic waves pass through the interface of different media, we elaborate the physical mechanism of frequency shift and find the characteristics of the GPR signal of nonmetallic pipelines in different spaces. It has practical significance for the GPR detection of nonmetallic pipelines.