Abstract: Lining defects in the hydraulic tunnels can affect the normal use of the tunnels, and thus affect the safe? ty and durability of water conservancy projects. This paper establishes a two-dimensional GPR forward model of tunnel lining based on the finite difference time domain (FDTD) method to investigate the GPR response characteristics of five types of tunnel lining defects: lining cavity shape, filling medium in the lining cavity, disengagement between the primary lining and secondary lining, uneven lining and detachment between the reinforcement and lining concrete, and summarizes the characteristics to identify each lining defect. Combined with a flood and sand discharge tunnel of a reservoir, using the obtained GPR spectrum response characteristics of different types of lining defects in the hydraulic tunnel, the multiple defects in the tunnel are identified and the lining structure and function are evaluated. The study results show that by using the FDTD method it can establish effective GPR models and provide a reliable basis for the interpretation of GPR images and the determination of lining defect types in hydraulic tunnels.