Gelatin-based hydrogels have multiple advantages because of their biocompatibility, biodegradability, and non-toxic properties. The feasibility of using gelatin-based hydrogels could be greatly improved if they could overcome the weak mechanical property for application. Here, strong fish gelatin hydrogels were prepared by introducing κ-carrageenan to gelatin solution to form structure preliminarily enhanced gelatin hydrogel, followed by immersing in 10 wt% concentration of K₂SO₄ solution. The modified hydrogels have an extraordinary hardness (21.22 N) and storage modulus (>10⁴ Pa) when carrageenan amount was 0.8 wt%, superior to those of unmodified gelatin hydrogel. The effects of carrageenan concentrations on thermal stability, water distribution and microstructures of hydrogels were systematically evaluated, and the possible strengthening mechanism was also discussed. This article provided a simple method for the preparation of gelatin hydrogels with strong structure and the modified hydrogels can be used in foods, materials and other fields.