A case study of the Wusong Jetty during long-term service is presented. Based on the results of field ultrasonic testing, laboratory strength testing, ground-penetrating radar testing, and underwater detection and measurement by divers, the degrees of degradation of sensitivity factors, including the concrete structure for the protective face, the internal rockfill structure, and the variation of the slope and evolution of erosion/deposition on both sides of the jetty, were evaluated. The effect of degradation of the four sensitivity factors on the deformation and stability of the jetty slope was investigated using the finite element numerical simulation method. The results show that the elastic moduli of the concrete structure for the protective face and internal rockfill structure exhibit negative exponential relationships with the maximum horizontal displacement of the jetty; the safety factor of the jetty decreases linearly with the increase of the slope, approximatively; and the maximum displacement of the jetty increases linearly with the increase of elevation induced by deposition on the left side and erosion/deposition on the right side of the jetty. The phenomena should be given more attention in design of schemes for reinforcement and maintenance in practice.