This study aimed to extract the active substance from Laminaria japonica (L. japonica) and evaluate their composition and hypolipidemic activity. The alcohol extract of L. japonica (LJA), water extract of L. japonica (LJW), enzymic hydrolysates of L. japonica (LJE) and dietary fiber extract of L. japonica (LJDF) were prepared by using different polar solvents combined with ultrasound-assisted alcohol extraction, water extraction, complex proteolytic enzymes and sodium carbonate acid, and their composition was analyzed. The above four active substances were mixed to obtain the total active substances of L. japonica (LJM). This study adopted C. elegans as a model organism to investigate the effects of bioactive compounds derived from L. japonica on lipid metabolism-related biochemical parameters and Messenger RNA (mRNA) transcription levels. The result revealed that the main components of LJA included choline, phospholipids, pyrimidines, terpenoids, arachidonic acid, steroids, polyphenols and disaccharides; LJW mainly consisted of two polysaccharides with molecular weights of 368.0 kDa and 1.0 kDa, with monosaccharide composition of glucose, galactose, fucose, mannose and arabinose; LJE mainly consisted of protein active peptides, containing six proteins (YP_006639117.1, AIW62928.1, WDS74817.1, WDS74887.1, QBF51285.1 and ABB80121.1), 43 peptides; LJDF mainly consisted of two polysaccharides with molecular weights of 717.073 kDa and 11.502 kDa, with monosaccharide composition of mannose aldehyde, gulonose aldehyde, galactose and fucose. The result of C. elegans experiments demonstrated that the L. japonica active substances significantly reduced triglyceride (TG) and malondialdehyde (MDA) levels (P<0.01). Meanwhile, these substances also increased total superoxide dismutase (T-SOD), catalase (CAT) and glutathione peroxidase (GSH-Px) levels (P<0.01). At the mRNA transcription level, the L. japonica active substances significantly upregulated genes NHR-49, FAT-5, FAT-6, FAT-7, DAF-2, and DAF-16 (P<0.01). Conversely, the substances significantly downregulated genes MOD-1, ACS-2, and AGE-1 (P<0.01). This suggested that the L. japonica active substances improved C. elegans lipid peroxidation and reducing lipid accumulation in C. elegans through regulating fatty acid β-oxidation (NHR-49, MOD-1 and ACS-2), fatty acid synthesis (FAT-5, FAT-6 and FAT-7) and insulin (DAF-2, AGE-1, DAF-16) signaling pathways.