Abstract: The cavity is one of the common defects in cement concrete pavement, and it is also the mainly hidden danger for driving safety. Ground penetrating radar can detect the cavity's scale and character by observing and researching the signal features reflected from high-frequency electromagnetic wave. With the advantage of confirming the cavity's existence and scale quickly and effectively, GPR has turned into a valid method in non-deconstruction defecting of road by far. Based on comprehensive analysis among GprMax2D simulating results, radar detecting statistics and verifying on site, the conclusion demonstrates that the radar image between aerated cavity and soil-ballasted cavity is pretty similar with an obvious feature:A group of evident strong reflection signal, arc-shaped axis in the same phase, will occur above the cavity; due to the distinct difference of dielectric constant, the phase between the reflection waves and direct waves is opposite for the soil-ballasted cavity while it is the same for the aerated cavity; the alignment of non dense area is irregular showing several irregular arc curves.