Porphyra haitanensis proteins (PHP) are natural proteins with various nutritional and biological values. This study was to analyze the composition, stability, and antioxidant activity of PHP before and after simulation gastrointestinal digestion (SGD). Caenorhabditis elegans was used as the model to investigate the functional activity and potential mechanisms of action of the PHP digestion products (PHPDP). The results showed that PHP contained 16 amino acids and exhibited high thermal stability (up to 80 °C), but dimerization or fragmentation occurred in environments with pH 3 and pH 11. After digestion, PHP released 212 bioactive peptides, which significantly enhanced its antioxidant activity and improved the resistance of C. elegans to heat, oxidative, and ultraviolet stress. Furthermore, PHPDP improved oxidative stress in C. elegans and extended its lifespan by increasing antioxidant enzyme activity and reducing malondialdehyde, lipofuscin, and reactive oxygen species levels. The mechanism of action of PHPDP likely involved regulating the insulin signaling pathway through daf-2/daf-16 and modulating the expression of oxidative stress regulators skn-1 and sod-3, thereby enhancing the organism's stress resistance and extending lifespan. This study demonstrated that P. haitanensis could serve as a reliable protein source in daily life and provided a reference for its development and application as a functional food ingredient.