The obligation of due diligence is a customary rule under general international law, which has been enshrined in numerous international treaties and legal documents, while its content and scope remain fragmented. With the accumulation of advisory opinion and judgment of international judicial bodies, such obligations in the United Nations Convention on the Law of the Sea has been uncovered, and its content and scope have gradually become clear. The obligation of due diligence is closely related to state responsibility. Nevertheless, as an obligation of conduct, the obligation of due diligence in the process of the exploitation of marine resources does not necessarily incur state responsibility in practice due to the flexible criteria in the application. Accordingly, the negative aspect of this obligation should not be exaggerated, and for those responsible states, the obligation would rather be an incentive. Under the guidance of the ‘strategy of maritime powers’, China should take necessary measures to regulate deep seabed mining activities as well as offshore fishing activities within its jurisdiction or control, so as to display the image of a ‘responsible great power’ in respect of marine environment protection.