Abstract: Total antioxidant capacity (TAC) is a crucial factor in assessing the antioxidant content of foods and determining the extent of oxidative stress in humans. In this study, a colorimetric sensor was constructed to provide sensitive and quick detection of TAC. The sensor was prepared using 3,3',5,5'-tetramethylbenzidine (TMB) as the chromogenic agent and Fe-N co-doped carbon (Fe-N/C) nanomaterials prepared by high-temperature pyrolysis strategy as the catalyst. The experimental results showed that Fe-N/C nanozyme represented a homogeneously dispersed dodecahedral morphology and had a high stability of recycling. In addition, the Fe-N/C nanozyme catalyzed the generation of reactive oxygen species (ROS) from hydrogen peroxide (H₂O₂), then facilitated the oxidation of TMB to oxidized TMB (oxTMB), finally, the changes of light signals can be recognized by naked eyes and detected by a UV-vis spectrophotometer. Kinetic analysis revealed that the K_m values of Fe-N/C nanozyme for the substrates were both lower than those of horseradish peroxidase (HRP) and most of the reported nanozymes, indicating that Fe-N/C nanozyme had a stronger affinity for the substrate and could reach maximum catalytic rates at lower substrate concentrations, which was favorable to the reaction. Since ascorbic acid (AA) is reductive, it can inhibit the catalytic oxidation process of the reaction, leading to a decrease in the corresponding blue product oxTMB. Therefore, AA can be quantified based on the response of absorbance values. Based on this principle, we constructed a sensor for the visual detection of AA, which showed an excellent linear relationship with the absorbance value for AA concentrations of 0.1-160 μ mol/L, with a detection line of 0.03 μ mol/L. Meanwhile, the TAC of kiwifruit, orange juice, and vitamin C tablets was evaluated using AA as the antioxidant model. The spiked recoveries ranged from 92.69% to 105.45% with the relative standard deviations (RSD) less than 2%, indicating that the colorimetric assay had excellent reliability and could be applied to the analysis of TAC in actual samples. In conclusion, we have successfully designed a simple, fast, highly accurate, and specific AA colorimetric sensor based on the excellent catalytic properties of Fe-N/C nanozyme, which breaks through the limitations of traditional detection methods. Most importantly, the nanozyme-catalyzed signal amplification strategy has high sensitivity, excellent specificity, simple operation, and low cost for the detection of the target, which is expected to be applied for the analysis of other targets and has promising applications in biosensing.