Most tunnels in China have different degrees of diseases, such as lining cracking, imperfect, lining containing voids and lining water leakage, which seriously influence the traffic safety. Ground penetrating radar (GPR) is used to detect the tunnel lining diseases rapidly and nondestructively. However, the interpretations for lining diseases of GPR detection results strongly rely on the experience of GPR users, which to great extent leads to missing and even wrong judgments. In this study the tunnel lining diseases are classified, and the typical types of tunnel lining diseases are modeled. Finite-difference time-domain (FDTD) method is used to perform forward simulations of typical tunnel lining diseases. For special conditions of water leakage of lining and interference induced by steel reinforcement, the frequency spectrum analysis is used to quantitatively analyze these diseases. Interpretation rules of GPR detection for typical tunnel lining diseases are summarized and further applied to a real project example. The results show that the forward simulations and interpretation rules of GPR detection for typical tunnel lining diseases are reliable.