Abstract: Sargassum fusiforme is a perennial large brown alga unique to the coastal waters of the northwestern Pacific. The young embryos of this alga play a crucial role in its sexual reproduction and are rich in various bioactive substances and characteristic fatty acids. To clarify the lipid structure characteristics, physiological functions, and fatty acid synthesis molecular mechanisms of the young embryos of S. fusiforme, we used purified and dried young embryos of S. fusiforme as the material, and employed UHPLC-Q Exactive mass spectrometry technology combined with Lipid Search software to analyze the lipid composition. Proteins related to fatty acid synthesis were identified by TCA/acetone precipitation, FASP digestion and database matching with Max Quant software, followed by KEGG pathway annotation. The results show that the lipids of S. fusiforme young embryos contained six classes including glycerophospholipids, sphingolipids, glycerides, pregnenolone lipids, fatty acids, and glycolipids, and 28 lipid subcategories. Among them, glycerol diesters (22.044%), phosphatidic acids (19.175%), and monohexosylglycerol diesters (18.526%) accounted for the highest proportions. The lipid composition of S. fusiforme young embryos included glycerophospholipids, sphingolipids, glycerides, pregnenolone lipids, fatty acids, and glycolipids, with 28 lipid subcategories. The fatty acid synthesis of S. fusiforme young embryos belongs to Type II FAS type, which is the classic plant fatty acid synthesis pathway. Starting from acetyl-CoA, fatty acids with different chain lengths were generated catalyzed by Fab series enzymes. This study initially reveals the lipid and fatty acid synthesis pathways in the young embryos of S. fusiforme, providing a fundamental theoretical reference for the analysis of photosynthesis mechanisms in the fertilized eggs and young embryos of Sargassum fusiforme and the high-value utilization of its resources.