Groundpenetrating Radar(GPR) is widely applied in engineering test, especially for tunnel lining structure. Based on Finite Difference Time Domain (FDTD) and Perfect Match Layer (PML) absorbing boundary condition, 900 MHz antenna is used to simulate singlelayer and doublelayer reinforced concrete with different spacing, depth and corresponding position. FK migration is used for image processing. The research shows: (1) within the range of effective detection, if the rebar spacing is larger and the depth is deeper, the interference signal is less and the response of the rebar is more obvious; (2) FK migration can identify singlelayer (or upperlayer) rebar, in the process of 2layer reinforcement simulation and migration, the effect of oblique is better than opposite.