Abstract: Electromagnetic wave tomography technology is widely used in geophysical prospecting. Drilling method is the common way to detect the inside structures of underground targets. To enlarge electromagnetic wave tomography application range and raise its universality, this paper aimed to do the study of electromagnetic wave tomography on overground targets. At first, discuss the detecting method and unite tomography imaging algorithm. Then, according to the different boundary conditions of overground targets detecting, this paper uses FDTD method to simulate and analyze the distributing stage of electro-magnetic wave, makes clear the extract method of primary wave, and also gives out zero point exact method of primary arrival wave. At last, experiment results verify the effectiveness of electromagnetic wave tomography technology aimed on overground target.