We carry out the ground penetrating radar forward modeling for three layers of air，cofferdam and silt on the basis of finite difference time domain，and analyze the radar wave propagation process and typical receiving patterns.The results show that the radar wave propagates in the form of spherical wave in the medium，and the energy disperses gradually with the propagation distance. When it encounters the interface with different electrical properties，reflection and transmission will be generated.The interface between the riprap layer and the soil layer can be inferred according to the radar reflection and the continuous characteristics of the in-phase axis，and the weak abnormal areas such as suspected mud impregnation can be delimited according to the abnormal reflection of the waveform，reversal waveform and the slow wave velocity. However，the receiving graph is distorted by multiple echoes，which is easy to cause misjudgment.By applying the ground penetrating radar to the actual riprap cofferdam project and combining with the borehole survey，the distribution of riprap layer thickness and the anomaly in riprap body are found out.