In order to analyze the ELF(extremely low frequency) magnetic field of ships, the corrosion related current modulated by the ship shaft was equivalent to a timeharmonic HED (horizontal electric dipole). Based on Maxwell′s equations and boundary conditions of electromagnetic field, the magnetic vector potential in air was modeled, and then the magnetic field expressions of the HED were deduced. The propagation character of the magnetic field was calculated by fast Hankel transform. The magnetic field of carbon electrodes in the pool and Tibased electrodes at sea were measured. Result shows the practicality of the magnetic field and the validity of the model.