Abstract: As to the propagation of EMP, the earth is a natural obstacle, so it's very significative to research the laws of EMP propagating into earth, which can be used into the electromagnetic protection of cables and devices. In this paper, The propagation laws of High-altitude electromagnetic pulse (HEMP) is researched, and propagation of HEMP, LEMP and UWBEMP, HEMP into earth and above earth are compared. Results show that: the electric field of high frequency EMP in earth weakens rapidly with the increase of depth, strengthens with the decrease of earth conductivity and permittivity, and the pulse width increases with the decrease of earth conductivity, weakens with the increase of polarization angle and azimuth angle, but strengthens with the increase of pitching angle. But the electric field of low frequency EMP in earth weakens slowly with the increase of depth. Based on these laws, some electromagnetic protection measures are proposed. The research in this paper is helpful to provide guidance for the protection design of electronic devices in the earth.