The ocean plays a crucial role in the global cycling of mercury (Hg). Mercury undergoes complex migration and transformation in the ocean, involving multiple interfaces (e.g., air-water, land-water, water-sediment, and water/sediment-biota) and processes (e.g., adsorption/desorption, deposition/resuspension, oxidation/reduction, and methylation/demethylation). In recent years, Hg stable isotope methods have developed rapidly and greatly promoted the cognition of the biogeochemical cycling of oceanic Hg, from tracing the sources and processes to reconstructing the paleoenvironment and paleoclimate. Here, we overviewed the accurate measurement methods of Hg isotope compositions in different marine samples, summarized the reported Hg isotope data in seawater, sediment and biological samples from different oceans of the world, and comprehensively analyzed the mechanisms of oceanic Hg isotope fractionation and its applications in tracing marine Hg cycling. We found that i) the available data for Hg isotopes in the ocean are very limited, ii) the current mechanisms and processes fractionating Hg isotopes are mainly obtained on basis of freshwater simulation experiments, while the research on Hg isotope fractionation mechanisms based on seawater is quite lacking, and iii) studies on Hg isotope compositions of single Hg species, such as Hg of different chemical forms in sediments and methylmercury in seawater/sediment/biological samples, are still scarce. As a result, large uncertainties existed in the accurate source apportionment of oceanic Hg. Moreover, it is difficult to establish a global marine Hg isotope conversion model, and precisely verify and restrict the key processes and corresponding fluxes of global Hg cycling. In the future, it is necessary to i) further enlarge the oceanic Hg isotope database, such as strengthening the in-situ or real-time measurement of Hg isotopes in seawater and important input sources like marine volcanic hydrothermal system and submarine groundwater system, ii) well investigate the mechanisms of Hg isotope fractionation during potential biogeochemical processes, especially those processes occurring in interfaces (such as sedimentation and bioaccumulation), and iii) deeply clarify the sources, migration and transformation of oceanic Hg using stable isotope methods, especially the origin and transmission of methylmercury, so as to provide basic data and theoretical support for controlling marine Hg pollution and understanding the global cycling of Hg.