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��