This study investigated the progressive structural changes of fish gelatin in thermally reversible (TR) and irreversible (TI) states, formed through microbial transglutaminase (MTGase) cross-linking during in vitro gastric digestion. The focus was on dynamic structural changes and gastric digestion characteristics. Free amino content and SDS-PAGE analyses showed that both TR and TI groups were hydrolyzed into smaller fragments by pepsin during digestion. Surface hydrophobicity and endogenous fluorescence analyses revealed that hydrophobic groups in TR samples became embedded, whereas those in TI samples were exposed as digestion progressed. The microstructure of fish gelatin varied states and changed over digestion time. Two digestion patterns were observed: the fitted curves of TR groups exhibited initially increasing that later stabilized, whereas the TI samples followed an S-shaped curve with three distinct stages-an initial stable phase, followed by a rise, and concluding with a final stabilization. A schematic model illustrated the dynamic structural changes of fish gelatin in different states during simulated gastric digestion. This study provides a basis for the development of high-quality collagen supplement products.