Abstract: When Ground Penetrating Radar is applied in actual projects, medium parameter (the permittivity is main) is a necessary data for users to position the depth of underground targets and the thickness of medium layers correctly. An inversion algorithm for underground medium parameter estimation based on MUSIC spectrum estimation is put forward in this paper. After estimating the time delay and amplitude of LFMCW GPR echo, the accurate medium parameter is worked out and then the right medium structure underground is gotten in succession. The simulating results show that the new algorithm improves much on the precision comparing with traditional electromagnetism inversion algorithms.