Identifying the patterns and ecological determinants of riverine community composition remains a key goal to understand community assembly, biological conservation, and environmental management. However, the patterns and potential causes of beta diversity variation and species replacement in riverine fish metacommunities are largely scarce at tropical islands. In this study, we investigated the environmental and spatial factors influencing beta diversity of local fish assemblages across several hydroligcal units of tropical rivers in the Hainan island, China. By collating 189 standardized fish samples during the low-water season across eight hydroligcal units, we tested the hypothesis that environmental factors infuence tropical fish assemblages beta-diversity more than spatial factors. We found marked consistency in high beta-diversity across hydrological units that influenced by species turnover rather than nestedness. Aligning with our hypothesis, environmental parameters alone explained a higher fraction of variation in beta-diversity than did spatial factors alone or shared environmental and spatial factors, implying that environmental filters constrain species distributions and abundances. In conclusion, our study suggested that conservation of fish diversity in tropical rivers requires maintenance of both local-scale habitat condition and riverine connectivity.