Abstract: The ancient building base (ABB) was almost constructed with brick and clay, and the structural ddfects induced by disrepair tend to be a serious problem. In this study, the ground penetrating radar (GPR), surface wave exploration (SWE), soil electrical resistivity method (SER) and time domain reflector method (TDR) are used to detect the structural defects and water migration. The investigation results show that: (1) After the rain, the saturation degrees of rammed earth (about 3.0 m in depth inside the the brick wall and 1.0 m in depth around the built-in drain-pipe) are obviously higher than those of the other regions, demonstrating that the causes for water leakage, brick wall crystalline bloom and peeling are cracks in the the brick wall, cracks in drain-pipe and invalidation of waterproof layer; (2) The damage degree of base top (about 5.0 m in depth) is much more serious than that of the base bottom, and the damage degrees of four side walls are unevenly distributed; (3) Because of the rainfall infiltration, the water content of rammed earth at the base top has an increasing trend, and the water content at the base bottom is higher than that at the base top. The average SER at the upper parts of ABB is larger than that at the lower parts, showing a slow decreasing tendency whether it rains or not. It is indicated that the water in the rammed earth migrates from the top to the bottom. This study can provide a set of research methods for exploring the defects formation mechanism of ABB.