To investigate the detection and recognition of underground cavity, a novel modified back projection algorithm based on the linear current radiation model of basic dipole is presented. The underground dielectric is analyzed, and the imaging model of underground cavity is built which the measured data is obtained through the half space finite difference time domain (FDTD). The computation formula of time delay compensation caused by the dielectric mutation is derived. The imaging result validates the accuracy of the modified back projection algorithm, and the effect of target structure to imaging is analyzed. The imaging of underground cavity is then studied. As well, the 3D imaging of cavity is provided to strength the ability of modified back projection algorithm.