In order to identify subgrade void disease quickly, the time domain finite difference method was used to numerically simulate the radar wavelength of the void disease's differences in size, length, location, and fill. Additionally, image enhancement techniques and complex signal analysis methods were applied to extract the instantaneous attributes of the radar wave field, such as amplitude, phase, and frequency. The radar wave field response characteristics of different debonding geologies were analyzed. The results show that the information concerning the upper interface of the subgrade void disease and the medium type of the void area can be calculated from the instantaneous amplitude images of the geological radar for subgrade void disease and single-channel wave normalized amplitudes. According to the instantaneous phase image characteristics of air-filled and water-filled void disease, the interlayer information of roadbed structure and the location and boundary of void disease area can be judged. According to the instantaneous frequency images of water-filled void disease and air-filled void disease, the positions of the high-frequency and low-frequency electromagnetic waves can be judged. In addition, according to the transient frequency images of single-channel waves of water-filled void disease, the location of forward electromagnetic waves can be obtained and used as the basis for judging the locations of the lower interface of void disease. The research results can provide a theoretical basis for the intelligent identification and rapid diagnosis of road diseases.