Thermally reversible and irreversible fish gelatins induced by different microbial transglutaminase (MTGase) contents (0.00–0.21%, w/v) were prepared and evaluated in cross-linking degree, gelation-melting transition temperature and gel strength. Moreover, modified fish gelatin was digested by in vitro digestion model, and produced collagen peptides were analyzed by molecular weight distribution, Tricine SDS-PAGE and HPLC MS/MS. With MTGase concentration increasing, cross-linking degree increased from 0.00% to 77.67%. Meanwhile, gelation and melting temperatures gradually increased, and then a thermal irreversible gel appeared when MTGase concentration reached to 0.15%. Regardless of thermally reversible or irreversible state induced by MTGase, fish gelatin could be hydrolyzed in vitro gastrointestinal digestion. The results of free amino content analysis showed that the greater cross-linking degree of gelatin, the more obvious delayed effect of digestion, especially when the gelatin was in a thermally irreversible state. However, the digestibility of gelatin increased with the increase of MTGase concentration. When the gelatin changed to a thermally irreversible state, the improvement in digestibility was very significant. More interestingly, after digestion, the types of collagen peptides increased first and then decreased with the increase of MTGase. When MTGase concentration was 0.06%, the types of collagen peptides were the largest, reaching 708. In addition, MTGase cross-linking with appropriate concentrations increased the type of hydroxyproline-containing peptides. The results showed that MTGase was beneficial to delay the digestion of fish gelatin and release a variety of collagen peptides, which would be significant to improve the quality of fish gelatin-contained collagen products and promote their application. Display omitted •In vitro digestion of fish gelatin in both thermally reversible and irreversible states was studied.•When gelatin gradually transformed into a thermally irreversible state, the delayed effect of digestion was more pronounced.•The stomach digestibility of fish gelatin significantly increased with increasing MTGase concentration.•Appropriate MTGase concentrations increased the types of collagen and hydroxyproline-containing peptides.