Abstract: Marine geohazards pose a major threat to the population and marine economic development of coastal cities. The expansion of construction of coastal port channels, submarine pipelines, optical cables, and marine platform foundations means that the risk of marine geohazards has further increased. Large-scale but uncommon disaster events such as submarine volcanic eruptions and tsunamis have attracted most of the public attention and media coverage and prompted policy adjustments to prevent and resolve disaster risks. However, small-scale but more frequent primary hazard events such as submarine gas leakages and submarine landslides, which can have severe localized impacts and are highly susceptible to transformation into hazard chains leading to worsening hazard events, are largely underappreciated by the public. To date, most features of marine geohazards can be detected and identified, but it is difficult to monitor effectively with existing technologies. In-situ monitoring of marine geohazards requires more demanding technical capabilities, especially in-situ monitoring of sudden-onset marine geohazards. The review first introduces the significance of in-situ monitoring of marine geohazards and the challenges of technology development, then summarizes and discusses the monitoring elements of marine geohazards, focuses on the application of marine geohazards monitoring technology and equipment, and analyzes and discusses the risk assessment and early warning of marine geohazards. Finally, summarizes outlooks on the marine geohazards in-situ monitoring technology and its application. The review aims to analyze and summarize some core technologies and key issues that need to be solved urgently in the monitoring technology and equipment of marine geohazards and their applications, in order to provide reference for the development and application of this technology.