In order to explore the structure of polysaccharide from Caulerpa lentilifera (CLP) and its effect on SH-SY5Y cells damaged by Aβ1-42 induction, an acidic polysaccharide component, CLP-1, was obtained through the separation and purification of CLP. The monosaccharide composition, molecular mass, glycosidic bond configuration, spatial conformation and microscopic structure of CLP-1 were characterized. At the same time, an SH-SY5Y cell damage model induced by Aβ1-42 was used to study the anti-Alzheimer's disease (AD) activity of CLP-1. The results showed that, the mass average molar mass of CLP-1 was 3.98×10⁶Da, and it was mainly composed of galactose, glucose, xylose, and mannose, with the molar ratio of these four components was 48.1∶1.0∶23.2∶46.1. Infrared spectroscopy analysis showed that CLP-1 had sulfate group substitution and existed in the form of pyranose. The results of nuclear magnetic resonance analysis showed that CLP-1 had two types of glycosidic bonds:α-glycosidic bonds and β-glycosidic bonds. Congo red experiment analysis showed that CLP-1 had a triple helix structure. The results of atomic force microscopy analysis showed that CLP-1 had a highly branched structure, and the results of scanning electron microscopy analysis showed that CLP-1 had a loose structure mainly consisted of spherical particles and rods-like shapes. The results of in vitro activity analysis showed that CLP-1 exhibited DPPH radical scavenging activity and suppressed Aβ1-42 aggregation. The results of cell experiments indicated that the morphology of SH-SY5Y cells pretreated with CLP-1 at a mass concentration of 0.05mg/mL was relatively intact. The cell survival rate increased by 20.03%, the level of total antioxidant capacity increased by 56.23%, the activity of catalase, glutathione peroxidase, and superoxide dismutase increased by 63.19%, 63.48% and 65.80%. The level of malondialdehyde decreased by 22.13%, the mitochondrial membrane potential level increased by 14.80%, and the level of reactive oxygen species dropped by 24.84%. The results showed that CLP-1 had good antioxidant and anti-AD activities. The purpose of this study was to provide a theoretical reference for the development of anti-AD functional foods. 〖