Islands are special regional units with unique development models and pathways and because of their locations, resources and environmental advantages, play an important role in the process of national modernization. Here, industrial coefficient variation and shift-share methods were used to explain the evolution of island county economies and the competitiveness of their industrial structures. We found that industrial structures in island counties have undergone a complex evolution, have not followed a pure levo-rotation mode or dextrorotation mode, and have experienced mixed evolutionary modes. The industrial structures in island counties are relatively simple, and the evolution of industrial structures in island counties has an apparent leaping feature. Last, the economic development of 12 focal island counties were divided into five categories in accordance with analysis of their economic development: Competitiveness Strengthened and Structure Optimized; Competitiveness Strengthened but Structure Declined; Competitiveness Weakened but Structure Optimized; Competitiveness Weakened and Structure Declined; and Structure Relatively Stabilized.