Abstract: The exploration of solution cavity is essential in the construction of pile foundation that results from the solution cavity and crack. Traditional method of solution cavity exploration is mainly based on resistivity method. However, depth is limited and boundary of solution cavity can't be obtained in this method. Additionally, drilling method can only explorer the karst cavity in an infinitesimal area. Electromagnetic Wave Computer Tomography (EWCT) is a developing geophysical method based on characteristics of spreading of electromagnetic wave in different medium. This paper mainly aims to substantiate the effect of karst cavity exploration with the method of EWCT. Result obtained by EWCT has been compared with kinescope and drilling data in an exploration of karst cavity in a huge bridge foundation. The result shows that EWCT is an effective method for karst cavity exploration.