In order to solve the problem of targets detection underground, an imaging algorithm is proposed based on the model of electric quadruple radiation. The lack of back-projection algorithm is analyzed qualitatively, a mathematical model of the underground target imaging, which proceeds from the effectiveness of underground targets imaging and delay characteristics of electromagnetic wave propagation, is built by improving the model of electric quadruple radiation, and the implementation process of the algorithm is given based on the derivation of time-delay compensation formula. MATLAB simulation verifies that the improved back-projection imaging algorithm is effective in detecting target underground and corrective in realizing the time-delay compensation. Then the algorithm is applied to the imaging of holes and landmines hided in the rock soil mass as well as to the urban pipeline underground, the effect of undulating ground and noise to imaging results is also analyzed.