Excessive lipid accumulation, a hallmark characteristic of high-fat diet (HFD)-induced obesity, has become a worldwide challenge, necessitating the exploration of secure and efficacious natural products for its intervention. In the present work, a polysaccharide (MCP) was extracted and purified from Mesembryanthemum crystallinum L., a novel halophyte, and its physicochemical properties, in vitro fermentation characteristics, lipid-lowering activity, and underlying mechanisms were systematically investigated. Physicochemical analysis revealed that MCP is an acidic polysaccharide, with galacturonic acid as the predominant monosaccharide component, broad molecular weight distribution, and a porous structural morphology. In vitro fermentation experiments demonstrated that MCP could be effectively utilized by human fecal microbiota, significantly promoting the yield of short-chain fatty acids (SCFAs), particularly butyrate at high concentrations, which outperformed inulin. 16S rDNA sequencing uncovered that MCP optimized microbiota composition by enriching SCFA-producing beneficial bacteria (Prevotella_9, Faecalibacterium) while suppressing opportunistic pathogens (Megamonas, Escherichia-Shigella). Metabolomic analysis of fermentation broth revealed that MCP significantly affected microbial glycerophospholipid metabolic pathways. Experiments in Caenorhabditis elegans (C. elegans) confirmed that MCP inhibited HFD-induced lipogenesis, which was linked to the regulation of the nhr-49/sbp-1-mediated lipogenesis pathway. For the first time, using an antibiotic-induced microbiota depletion model in C. elegans, the lipid-lowering effect of MCP was observed to disappear, suggesting a potential role of the gut microbiota in mediating this effect. This investigation establishes a scientific basis for MCP as a novel prebiotic or dietary supplement for managing obesity-related lipid accumulation.