Twodimensional forward model of hollow beneath the steel fabric of tunnel lining and behind tunnel lining is established based on finite difference time domain (FDTD) method and finite element method (FEM). The modeling results are compared with actual case. The results show that: 1） The higher the center frequency of the radar antenna is, the higher the resolution is, and the better the interpretation is. 2) The smaller the density of the reinforcement is, the better the detection effect of radar is. The accuracy and interpretation precision of ground penetrating radar （GPR） detection of tunnel lining can be improved by combining the forward simulation method and site monitoring.