The uncompactness grouting of box girder prestressed pipes results in the instantaneous failure of the prestressed stress, which causes the damage of the sudden fracture and collapse of the girder body.In order to detect the non-compactness of grouting in box girder prestressed pipes more accurately, a physical model of cavities in box girder prestressed pipes was established, and the imaging characteristics of reflected wave signals under two-dimensional and three-dimensional ground penetrating radar detection of reinforcing bars, partial grouting and water-filling conditions were analyzed. Based on the finite element method calculation formula of improved Sarma absorbing boundary, the forward simulation of cavities in box girder prestressed pipes with different sizes were carried out by using the Matlab program.Through the comparison and analysis of model test and numerical simulation,the results show that: the three-dimensional detection of ground penetrating radar can identify the distribution of reinforcement and cavities, and more comprehensively reflect the morphological characteristics of different diseases.The finite element simulation analysis shows that the time of echo signal, echo intensity, hyperbolic curvature and elongation are all related to the geometrical size of the cavity. The results of both simulation and test detection are "arc", which verifies the feasibility of the numerical simulation method and provides guidance for practical work.