Due to the special geographical location and unique ecosystem of islands, appropriate development and protection of islands is important for promoting the sustainable development of islands. By using the Pingtan Islands as an example, this study constructed a system of suitability evaluation indexes for marine ecological space, marine agricultural production space and marine construction space under the principle of land–sea coordination. To evaluate the spatial suitability of the sea area around islands, a GIS spatial analysis method and a hierarchical analysis method (AHP) were used, which provided useful references for the integration of land and sea and for developing islands sustainably. The research results showed that the suitability area of ecological space in the Pingtan Islands accounted for around 64.3%, the construction space accounted for around 2.1% and the agricultural production space accounted for 33.5%.