<p>This study examined an instability event of a gravity block wharf on the southeast coast of China. Two scenarios were considered: treating the wharf as an individual structure and as part of an integrated wharf–breakwater system. Using original design parameters and data from a second geological investigation after the failure, a numerical model of the breakwater was developed, and sectional finite element stability analysis was performed to explore the mechanism of instability. Results showed that the wharf failure was not due to wave-induced stress concentration at the outer sea corner causing local breakwater damage. Variations in design loads had little impact on displacement, and failure modes under different loads were similar. The displacement loading method was applied to identify failure patterns. The structure exhibited overall subsidence, separation between the wharf and rear slope dike, tilting of the wharf toward the dike, upheaval of the front foundation bed and soil, tilting of the slope dike toward the wharf, and the formation of a sliding zone between the structure and foundation. These behaviors indicated that the actual failure mode corresponded to instability caused by insufficient bearing capacity of the soft soil foundation. Thus, the observed instability was consistent with typical failure characteristics of gravity structures under weak foundation conditions. The study provides theoretical reference for risk assessment and teaching in similar engineering projects.</p>