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Lining void is a common defect in tunnel construction, which might lead to the deterioration of tunnel structure and reduce tunnel safety. It is necessary to detect the tunnel lining in the stages of construction and operation. In this study, a three dimensional (3D) model of tunnel lining and surrounding rock is established based on finite difference time domain (FDTD) method, and the application of 3D GPR (ground penetrating radar) in lining detection is simulated forward to obtaine the 3D GPR data of void. The response characteristics of void in different measuring lines are analyzed, the influence of lining void on electromagnetic waveform and wave field distribution are discussed. Finally, the reliability of the forward modeling result is verified by a tunnel project. The results show that the electric intensity of void area increases as the measuring line is close to the center, and the three dimensions of void can be detected according to this change. In addition, the electromagnetic wave is reflected many times in the void, causing the concussion in the surrounding rock layer. Besides, the metal parts will cause strong interference to the signal of the void, and the extreme values of the electric field are metal-free parts, single-layer steel and double-layer steel in order from large to small. This study will provide valuable reference for promoting the application of 3D GPR in lining void detection and improving the accuracy of detection. [ABSTRACT FROM AUTHOR]