A survey method of the cross hole seismic CT technology was introduced to karst exploration in a section of metro line 4,Nanjing City. A Matlab program was compiled to realize the function of velocity tomography between two boreholes through solving the least squares solution of the linear sparse matrix equation, generating 3D grid and wave velocity contour map. The position and size of karst caves along with whether or not filled were determined according to the range of low velocity zone and seismic waveform characteristics. The result of checkerboard resolution test directly shows that the major characteristics of CT image are reliable. Through interpreted results were consistent with the drilling data and ground penetrating radar (GPR) image. Thus, this geophysical method had a high reliability on detecting karst caves. And advance forecast on karst developing status could be finally achieved.