To evaluate the failure risk of the fish cage working at sea, this study analysed the yield and fatigue phenomena of cage floating structure under excessive and long-term wave loads. According to the mechanical tests, the mooring forces of C80 float in normal and extreme sea conditions were 27.41 kN and 86.13 kN, respectively. The S – N curve of HDPE floating material was described by S = −11.93 ln(N) + 29.076. Based on finite element analysis, the C80 failed when the overall horizontal load was 3000 kN or wave height was 23.4 m. The main floating structure showed sheared failure when twisted 5 m for the four-point displacement-load. The failed and fatigued areas were mainly in the mooring points, and the stress concentration mainly occurred at the cracked parts. The C384 cage construction and site test were conducted in the East China Sea. The specifications of the floating structure were enhanced, and the circular curvature and assembling clearances increased. This study could provide guidelines on the structural design and optimisation of the HDPE super large fish cage. • Float yield and fatigue analysis under excessive or long-term wave. • Fatigue S – N curve of HDPE float was described by S = −11.93ln(N) + 29.076. • C80 failed when the overall load was 3000 kN or wave height was 23.4 m. • Float showed sheared failure when twisted 5 m for four-point displacement-load. • Guidelines for the design and optimisation of HDPE super large fish cage. [ABSTRACT FROM AUTHOR]