As a geophysical method based on the reflection of electromagnetic wave, ground penetrating radar (GPR) is interfered by the underground metal pipeline in underground disease detection of urban road. In order to find out the interference region around the metal pipeline, the authors deduced the calculating formula of the interference region based on the resolution of GPR. After the numerical simulation and the physical model simulation, the authors found that there is an interference region with a vertical height of the electromagnetic wave length beside the metal pipeline. When the underground disease is located in this region, its reflection wave group will be disturbed, which is not conducive to the interpretation of disease's attribute and scale. And when the underground disease is located under the metal pipeline, it cannot be detected by the current single type antenna.