Abstract: This study employed fish gelatin (FG) as the raw material and modified it using ultrasound-assisted phosphorylation (UAP) technology to investigate the effects of different ultrasonic power levels (50, 100, and 200 W) and processing temperatures (50, 60, and 70 ℃) on the digestive and fermentative properties of FG. The in vitro digestion model was used to systematically study the digestion characteristics of FG before and after UAP modification. Concurrently, an anaerobic intestinal fermentation model, using intestinal microorganisms as the primary fermentative agents, was applied to explore the impact of this phosphorylation modification on the fermentation characteristics of FG. The results showed that fish gelatin modified by UAP showed a reduced digestion rate in the gastrointestinal digestion stage. Specifically, the digestion products from FG treated at 100 W and 70 ℃ demonstrated significantly enhanced antioxidant activity, which had potential advantages in resisting oxidative stress compared with unmodified FG. UAP treatment significantly altered the fermentation characteristics of FG, particularly with notable differences in gas production, pH shifts, and short-chain fatty acid (SCFA) generation. Furthermore, 16S rDNA sequencing analysis revealed that UAP treatment markedly enriched the microbial diversity in the fermentation broth, including an increased abundance of probiotic bacteria. The findings demonstrate that ultrasound-assisted phosphorylation (UAP) can improve the digestive properties, antioxidant activity, and intestinal microbiota fermentation functionality of FG, providing a scientific basis for its development and application in the food industry.