In this paper,the ground penetrating radar is applied to the road detection,and the field test is carried out. The radar test results are compared with the results of core sampling. The location of the interface,the thickness of structural layers,and the existing diseases are analyzed. The Hilbert transform of radar raw data is used to obtain instantaneous amplitude,instantaneous phase and instantaneous frequency,then,the interface position of the road structure layer is emphatically analyzed. The results show that the ground penetrating radar can detect the thickness and the uneven settlement of the road structure layer. The accuracy and credibility interpretation of the radar image are improved by secondary processing.