Nowadays in China, with increases in the lengths of new highways and the resultant increasing construction times, many personal injuries have occurred as a result of the structural defects such as track-plate fracture, through-cracks, layer alienation gap, and pulping. Investigating the damages of highway structures is an important and difficult subject, especially for defect diagnosis in a structure's support plate and subgrade surface layer. As various current subgrade defect detection methods are limited and ineffective, how to detect faults quickly and accurately is an important issue in civil engineering. In light of the permanent stability of the subgrade of an operational highway, a full-wave field imaging method was developed to detect defects in a highway subgrade. This detection method was a non-destructive testing method based on the characteristics of high-speed highway structures and the existing level of detects. The detection system was composed of a data acquisition system and an imaging system. The data acquisition system utilized a single-point source and a single receiver, in which three-dimensional full-waveforms were created, received, and recorded. First, to evaluate the feasibility of the method to reveal defects, a 3D numerical simulation by a finite-element dynamic method was adopted based on the theory of elastic wave propagation. Using a 3D numerical model, the characteristics of a full-wave field within some defects were studied, then the amplitude and frequency spectrum of the multi-directional response waveforms were analyzed. Based on the synthetic image, the defects were revealed and their locations and sizes agreed well with those of the numerical model. Second, a real full-scale highway subgrade model with pre-setting defects was casted, and relative tests were carried out on it. The size and location of defects revealed by the field experiments were similar to that of the actual model. The accuracy of the full-wave field imaging method proved to meet the detection requirements, such as easy maneuverability and rapid speed of detection. It is concluded that the full-wave field imaging method has high potential in providing quick and effective detection of defects in highway subgrades.