The invention relates to an offshore oil gas field exploiting system which is characterized by comprising a crude oil storage tank arranged on a seabed, wherein the inlet of the crude oil storage tank is communicated with an oil gas exploiting device by virtue of an oil delivery pipe; the outlet of the crude oil storage tank is communicated with a crude oil output float bowl which floats on the sea surface. According to the offshore oil gas field exploiting system, the crude oil storage tank is transferred to the seabed from a platform, so that the area, volume and load bearing of the platform are greatly reduced, meanwhile various problems brought by the overlarge volume and load bearing are also solved, and the crude oil gas storage amount is also greatly improved; in addition, the crude oil storage tank is positioned in water, so that the occurrence possibility of potential safety hazards such as fire hazards is greatly reduced, furthermore equipment such as fire fighting devices can be omitted, and the whole system is simpler and more reliable.