A mathematical model is proposed to calculate the electric fields induced by the motion of metal body across the earth's magnetic field, according to deriving the expressions of electric fields induced by the vertical and horizontal DC current under seawater using the mirror image method. The magnitude and spatial distribution characteristics of the electric fields were analyzed, the results show that in the 30m depth, the magnitude of the electric field induced by a 20m′s long metal ellipsoid sailing 10m below water surface at the velocity of 4m/s is nV/m, and apparent characteristics can be noticed in its spatial distribution.