Lining cavities are common diseases in operating tunnel.Because of its complex medium inside

especially water filling

there are always difficulty and error in ground penetrating radar（GPR） detecting.Firstly

the numerical model of cavity and cavity water with different conductivities is established using MATLAB programming.Then GPR forward simulation is carried out with 2D finite difference time domain（FDTD） method.Through analyzing forward simulation map and its single trace

the response character of cavity and water is summed up.Appling GPR on detecting tunnel lining model

comparison is made between the test result and the forward simulation one.It is shown that the model test result is similar to FDTD forward simulation.The reflected wave phase of cavity is opposite to incidence wave.But phase of cavity water is the same as incidence wave.Furthermore

reflect wave of water is more obvious.Electromagnetic wave attenuation in water is too fast to identify the size of cavity.