This study investigated the water retention properties and the effects of ectoine on the storage quality of frozen sea bass fillets. Ectoine solution with mass fractions of 0.25 %, 0.35 %, 0.45 % and 0.55 % were prepared, using distilled water as a control, and compared with phosphate as a water retaining agent. Periodic assessments of water retention indices and storage quality were conducted during frozen storage. The results demonstrated that ectoine treatment reduced the thawing loss rate and freezing loss rate while improving the storage quality of sea bass fillets. Correlation analyses and molecular docking revealed the molecular mechanism underlying ectoine's enhancement of water retention. Chemical forces such as hydrogen bonding, hydrophobic interactions and ionic bonding between ectoine and muscles maintain water retention and inhibit quality deterioration.