The effect of ultrasound-assisted immersion freezing at 180 W (UIF-180) on the microstructure, quality and water distribution of porcine longissimus muscles during frozen storage was evaluated. The size of the ice crystals increased with extended storage time, and UIF-180 samples had a smaller size and uniform distribution of ice crystals. The thawing and cooking losses in the UIF-180 sample were significantly lower than that in air freezing (AF) and immersion freezing (IF) samples (P < 0.05). AF samples had a higher cutting force at 0 days. During the 60–180 days of the storage period, the cutting force of UIF-180 samples was significantly higher than that of AF and IF samples (P < 0.05). Low field-nuclear magnetic resonance results showed that UIF-180 decreased water migration during frozen storage. UIF-180 samples had significantly lower lipid oxidation and higher redness than that of the AF and IF samples (P > 0.05). Overall, UIF at particular powers is an efficient method in reducing quality deterioration of muscles during long-term frozen storage. •UIF-180 sample had small and uniform distribution of ice crystals during storage.•UIF treatment reduced the thawing and cooking losses and lipid oxidation of samples.•UIF-180 samples had a shorter T2 relaxation time during frozen storage.•UIF at certain powers is as an efficient method in reducing quality deterioration.