In this work， sea bass was used as a raw material to study the effect of ultrasonic-assisted freezing and low-temperature quick-freezing at the same temperature （-20 ℃ and -40 ℃） on the ice crystal morphology of sea bass fillet and the physicochemical properties and structure of fish myofibrillar protein during freezing at -18 ℃. The results indicated that the -40 ℃ ultrasound-assisted frozen samples had a fast freezing rate， small and uniform distribution of ice crystals， and the highest solubility and content of total sulfhydryl groups compared to the other three groups， and could effectively reduce the degree of myofibrillar protein oxidation. At the same time， it could also improve the thermal stability of fish protein and better maintain the structure of sea bass protein.