Ascorbic acid (AA), which is a well-known antioxidant, reacts with reactive oxygen species (ROS) inside the human body. It is crucial for the proper functioning of living organisms, however, the sensing of AA is still a huge challenge. Herein, a mixed Ce/Eu metal–organic framework [ZJU-136-Ce_1−xEu_x (x = 0.24, 0.36)] is designed and synthesized. Due to the specific redox reaction between the enediol group of AA and Ce⁴⁺, the fluorescence intensity of the ligand increases significantly with a simultaneous fluorescence quenching of Eu ions, resulting in a ratiometric fluorescent probe for detecting AA. It is worth noting that the ZJU-136-Ce_1−xEu_x (x = 0.24, 0.36) exhibited good luminescence properties and ratiometric fluorescence for AA determination. Furthermore, the regeneration cycle test shows that the mixed Ce/Eu metal–organic framework can be used as a potential ratiometric sensor for AA determination.