The niche and interspecific relationships between Sepiella japonica and major nekton in the waters of the Zhongjieshan Islands were analyzed by niche breadth, niche overlap and association coefficient (AC) to explore the niche of S. japonica in the ecosystem, according to the survey data of fishery resources in the Zhongjieshan Islands sea area of four voyages in spring (April) and autumn (October) from 2020 to 2021. The results showed that 21 major nektons in spring and 14 in autumn. The niche breadth values of S. japonica in different seasons were quite different. S. japonica belonged to a wide niche in spring and a middle niche in autumn. In spring, the niche overlap between S. japonica and major nekton was low. The species pairs with O_ik≥0.5 accounted for only 14.29% of the total. In autumn, the niche overlap between S. japonica and major nekton was high. The species pairs with O_ik≥0.5 accounted for 50% of the total. AC analysis showed that the proportion of species pairs with positive interspecific association (AC≥0) between S. japonica and major nekton species was higher, among them, 19.05% of the 21 species pairs in spring had significant positive associations. A total of 14 species pairs in autumn, 35.71% had significantly positive associations. Research showed that the resource utilization ability, environmental adaptability and interspecific association of major nekton species were different in different life history stages of S. japonica. Based on the niche perspective, the resource decline mechanism of S. japonica was further explored, which could provide ecological basis for the protection and restoration of S. japonica habitat.