It is well known subsea tunnel plays a significant role in promoting regional and national economic development. However, owing to their special service environment and construction difficulty, subsea tunnel projects exhibit pronounced characteristics of highly aggregated multi-dimensional risks and strong uncertainty in risk evolution during construction. Generally, it crosses fault zones, weak fractured rock masses, and deep overburden layers, and are typically subjected to the extreme condition of “a water body overhead”. Therefore, given the complex risk causation mechanisms, long action chains, and strongly coupled evolution processes in subsea tunnel construction, establishing a systematic risk assessment and dynamic control framework for multi-dimensional risk coupling has become a critical engineering challenge in subsea tunnel risk management. In view of these high-risk characteristics in a real case undersea tunnel in China, this lecture presents multi-dimensional risk assessment and its application in this tunnel. First, the fundamental principle, method and the coupling and cascading amplification of multiple risk losses are proposed. Then the probability prediction and application of various engineering hazards are introduced. Risk assessments for typical risks such as complex geological conditions and sudden water gushing, long-distance construction, mutual influence among three-line construction is conducted.