Abstract: Most of the underground pipelines in northwest cities and towns are laid in the natural and artificial formation of secondary loess media environment. When interpreting the radar detection signals of underground pipelines, engineers often overlook the interference caused by the dielectric inhomogeneity of secondary loess on the reflection signals of underground pipelines detected by ground-penetrating radar, thus causing interpretation errors. We established the inhomogeneous and homogeneous soil dielectric models with metal pipeline and plastic pipeline based on gprMax according to the environmental characteristics of secondary loess media for underground pipeline laying in Northwest China, and performed the positive evolution of the above models. On the basis of reflection signal image analysis, single channel reflection signal analysis, and amplitude spectrum analysis, we used engineering practice to verify the signal characteristics of metal pipeline and plastic pipeline laid in secondary loess medium. The results show that: the image signal of the inhomogeneous soil dielectric model is closer to the actual geotechnical dielectric model; the singular value method can improve the target discriminability of the measured signal by more than 2 times; the reflection peak of the metal pipeline is 3.7 times higher than that of the plastic pipeline; the calculated depth of the metal pipeline is shallower than the actual depth, and the calculated depth of the plastic pipeline is deeper than the actual depth; in the secondary loess medium, 200~400 MHz is the dominant frequency range of the amplitude spectrum of metal pipeline and plastic pipeline. The research results can guide the signal analysis and feature identification of metal and plastic pipes in the secondary loess environment.