Marine controlled source electromagnetic signal could be used in mineral resource exploration，reservoir appraisal and communicative technique in ocean． It's necessary to study the electromagnetic generated by MCSEM． The propagation of the electromagnetic fields from a controlled source in the marine environment was studied with virtual interface method combined with discrete complex image method． Transmitter of finite length current source is approximated by dipole ( HED) ． A three-layered model is accepted，with sea water as intermediate conductive layer under air and a relatively high resistive seabed as basement，possibly containing a hydrogen layer of higher resistivity． The electromagnetic fields in whole space thus computed show that: ( 1) the spatial distribution of field component depends on its type; ( 2) inline Ex component is more sensitive to reservoir layer than that in broadside; ( 3) The airwave affects marine electromagnetic ( MEM) exploration when sea water is relatively shallow; in the case of deep water MEM exploration，the airwave influence could be neglected; and ( 4) an appropriate frequency should be selected in order to balance the signal strength and electromagnetic induction effect．