The assessment of crab resources in Xiamen coastal waters can provide basic information for the sustainable utilization and management of crab resources in Xiamen coastal waters. Based on sampling data from bottom trawl surveys in the coastal waters along the Xiamen (24°21′﹣24°39′N, 117°54′﹣118°30′E) collected by shrimp trawls during May (spring), August (summer), November (autumn) in 2016 and February in 2017 (winter) (Fig. 1), species composition, composition of dominant species, species diversity and biomass distribution of crab in Xiamen sea area were studied. There were 32 species of crab, belonging to 12 families and 18 genera (Table 1), being identified in the survey area. The highest species diversity occurred in spring and lowest in autumn (Table 2). Of them 7 species are dominant, named Charybdis variegata, C. orientalis, Portunus sanguinolentus, P. hastatoides, Parthenope validus, C. japonica and Portunus pelagicus. However, temporal- spatial distribution of dominant species were obviously different, there were three in spring, four in summer, four in autumn and two in winter. Among them, C. japonica was dominant species in four seasons of the year (Table 3). The species richness index (D) was 1.46﹣4.09, the Shannon-Wiener index (H′) was 1.35﹣2.25, and the species evenness index (J′) was 0.59﹣0.73. The diversity index was the highest in spring and the lowest in autumn (Table 4). The average biomass was 101.56 kg/km2 in autumn and 24.61 kg/km2 in spring, which was the highest and lowest season in a year (Fig. 2), Jiulong River Estuary exhibited lowest crab biomass when compared to other sampling sites (Fig. 3). Again, the biomass also varied with water depth. The biomass of water depth above 10 m was smaller than that of water depth under or equal to 10 m (Fig. 4). Except winter, it was concluded that biomass of crab varied with the seasons, stations and water depth, which were closely related to Jiulong River runoff, water depth, water temperature and other environmental factors. Compared with 30 years ago, the number of crab species in Xiamen decreased by 45% and the dominant species have changed obviously (Fig. 5).