Abstract: Karst development in limestone areas holds great potential hazards to the urban construction, Using the conventional way of drilling, it is difficult to fully identify the characteristics and scale of underground Karst development in urban Karst exploration while cross-hole electromagnetic wave tomography can find out the state of development and extension of underground Karst in the work area. The forward electromagnetic wave tomography which is performed on the boreholes indicates the accuracy and feasibility of electromagnetic tomography for Karst investigation. Based on the examples of Karst exploration in metro rails, the electromagnetic wave tomography is applied, thus the two-dimensional absorption coefficient and the three-dimensional absorption coefficient images are retrieved respectively. By obtaining the absorption coefficient slices of different depths and horizontal spaces, the 3D spatial form of the underground Karst cave can be reconstructed, and detailed data for Karst exploration in construction of major projects can be provided.