Frozen　aquatic　products　quality　degrades　primarily　due　to　ice　crystal　formation.　This　study　developed　a　composite　cryoprotectant　(CC)　consisting　of　marine　Sargassum　fusiforme　polysaccharide　(SFP)　for　frozen　large　yellow　croaker　fillets.　The　optimal　formulation　of　CC　was　1　%　SFP,　0.125　%　tea　polyphenols,　and　2.25　%　food-grade　phosphate.　The　fillets　treated　with　CC　and　stored　at-18　degrees　C　for　80　days　showed　higher　salt-soluble　protein　content,　Ca2+-ATPase　activity,　total　sulfhydryl　content,　apparent　viscosity,　and　a　more　intact　microstructure　compared　to　control　group.　TCA-soluble　peptides　and　thiobarbituric　acid　value　were　lower,　indicating　improved　myofibrillar　protein　stability　and　overall　fillet　quality.　The　treatment　enhanced　water　retention　and　distribution　characteristics,　demonstrating　the　high　cryoprotection　efficiency　of　the　SFP-based　CC.　This　novel　cryoprotectant　shows　effects　comparable　to,　or　even　exceeding,　commercial　cryoprotectant　in　mitigating　ice　crystal-induced　damage,　making　it　a　promising　solution　for　frozen　aquatic　products　storage.