Abstract: The development characteristics of burial karst and sources of the karst fillings are very important for choosing the location of super-high buildings and bearing strata. This study employed the cross-hole seismic CT method to detect the caves and fracture zones of a foundation in Wuxi, and made indoor tests on samples of the karst fillings from the typical karst caves. Combined with the in-situ geological conditions, this paper presents a qualitative analysis of the scale, distribution and development characteristics of the karst and the sources of fillings which are more than 80 meters below the foundation. The results show that the karst is of a small size, the area of most caves is less than 2 m2, and there are many layers in the vertical direction. The rate of cave filling reaches 92.7%, and the caves are mostly filled with hard plastic mud which has a large bearing capacity and can be used as the bearing layer of foundation. XRD analysis of mineral composition shows that the minerals of different cave fillings are generally the same, consistent with those of the overlying bedrock. Thus, the denudation of overlying bedrock is the main source of these fillings. There is no large connective channel between the caves, and the underground water level has dropped below the caves. It is therefore concluded that these caves will not further develop and pose no threat to the stability of the foundation. The results of this study will provide reference for the site selection, design and construction of super-high buildings, karst detection, stability analysis of foundations and the treatment of unfavorable geological bodies in the Suzhou-Wuxi-Changzhou areas, which are located on deep karst foundation.