To explore the effects of a phosphorus-free water-retaining agent consisting of sodium bicarbonate, sodium citrate and sorbitol on the physicochemical properties and microstructure of sturgeon slices under repeated freeze-thaw conditions, sturgeon fillets were treated with the water-retaining agent or distilled water as a control before being repeatedly thawed and frozen up to five times. At each cycle, the thawing loss percentage, cooking loss percentage, pH, thiobarbituric acid (TBA) value, myofibrillar protein solubility, Ca²⁺-ATPase activity and texture characteristics were measured. Meanwhile, the microstructure was examined. The results showed that with the increase in the number of freeze-thaw cycles, the thawing loss percentage, cooking loss percentage and TBA value were significantly increased (P < 0.05), the waterretaining agent treatment plus quick freezing group exhibiting a slower rate of increase in these parameters compared with the water-retaining agent treatment plus slow freezing group and the control group. For all groups, pH decreased first and then increased, water content, myofibrillary protein solubility, Ca²⁺-ATPase activity, hardness and elasticity significantly decreased (P < 0.05), and the rate of decrease in these parameters was slower in the water-retaining agent treatment groups than in the control group. Cross-sectional images showed that the microstructure of sturgeon fillets could be maintained better by water-retaining agent treatment compared to the blank control group. These findings show that the water-retaining agent can inhibit the quality deterioration of sturgeon fillets under repeated freeze-thaw cycles.