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Increasing concern over the safety of consumable products, particularly aquatic products, due to freshness issues, has become a pressing issue. Therefore, ensuring the quality and safety of aquatic products is paramount. To address this, a dual-mode colorimetric–fluorescence sensor utilizing Ce-MOF as a mimic peroxidase to detect H₂S was developed. Ce-MOF was prepared by a conventional solvothermal synthesis method. Ce-MOF catalyzed the oxidation of 3,3’,5,5’-tetramethylbenzidine (TMB) by hydrogen peroxide (H₂O₂) to produce blue oxidized TMB (oxTMB). When dissolved, hydrogen sulfide (H₂S) was present in the solution, and it inhibited the catalytic effect of Ce-MOF and caused the color of the solution to fade from blue to colorless. This change provided an intuitive indication for the detection of H₂S. Through steady-state dynamic analysis, the working mechanism of this sensor was elucidated. The sensor exhibited pronounced color changes from blue to colorless, accompanied by a shift in fluorescence from none to light blue. Additionally, UV–vis absorption demonstrated a linear correlation with the H₂S concentration, ranging from 200 to 2300 µM, with high sensitivity (limit of detection, LOD = 0.262 μM). Fluorescence intensity also showed a linear correlation, ranging from 16 to 320 µM, with high selectivity and sensitivity (LOD = 0.156 μM). These results underscore the sensor’s effectiveness in detecting H₂S. Furthermore, the sensor enhanced the accuracy of H₂S detection and fulfilled the requirements for assessing food freshness and safety.