Rare earth up-conversion nanoparticles (UCNPs) have been used to construct fluorescent nanosensors because of their low toxicity, good chemical stability, and low background fluorescence. Core-shell UCNPs were prepared using the solvothermal method, and water-soluble core-shell UCNPs (Cit-CS-UCNPs) were obtained by surface ligand exchange with sodium citrate. The Cit-CS-UCNPs were used as the energy donor of the fluorescence sensor, and manganese dioxide (MnO2) nanosheets were used as the energy receptor of the fluorescence sensor. Based on the fluorescence resonance energy transfer (FRET) mechanism, a fluorescence nanosensor (Cit-CS-UCNPs-MnO2) was constructed for the detection of hydrogen dioxide (H2O2) and tert-butylhydroquinone (TBHQ) as food additives. The prepared nanomaterials morphology, structure, and properties were characterized by scanning electron microscopy (SEM), fluorescence spectrum, and ultraviolet spectrum (UV-vis). The effects of quenching agent concentration, incubation temperature, and incubation time on the detection performance of the fluorescence sensing system were investigated. According to the fluorescence spectra and UV-Vis experimental results, the maximum emission peak of Cit-CS-UCNPs is 654 nm. After the combination of Cit-CS-UCNPs and MnO2, core-shell UCNPs undergo fluorescence quenching. When H2O2 is present, the fluorescence of Cit-CS-UCNPs recovers. The results indicate that H2O2 and MnO2 nanosheets undergo redox reaction at this band, and MnO2 nanosheets are reduced to Mn2+, which gradually dissociates from the surface of Cit-CS-UCNPs. In the presence of TBHQ, the peak of the Cit-CS-UCNPs-MnO2 and TBHQ system was shifted to 253 nm, indicating that the redox reaction occurred between TBHQ and MnO2 nanosheets, and the FRET effect was reduced in the Cit-CS-UCNPs-MnO2 system, and the fluorescence intensity increased. It can be seen from the SEM results that MnO2 nanosheets are uniformly coated around Cit-CS-UCNPs, and maintain good dispersion in water, indicating that MnO2 nanosheets are modified on the surface of Cit-CS-UCNPs. The concentration of the quencher potassium permanganate (KMnO4) was optimized, and the results showed that when the concentration of KMnO4 is 10 mol·L-1, the quenching efficiency can reach 90%. The detection conditions were optimized, and the results showed that when the incubation time of H2O2 was 25 min, the redox reaction between MnO2 and H2O2 was complete. The fluorescence recovery value of Cit-CS-UCNPs-MnO2 was the maximum. The incubation time of TBHQ was 30 min. Under the optimal experimental conditions, the fluorescence intensity of Cit-CS-UCNPs-MnO2 has a good linear relationship with the concentration of H2O2 (0~1 000 μmol·L-1) and TBHQ (0~0.6 mmol·L-1). The optimal experimental conditions were maintained, and representative metal ions (such as K+, Na+, Ca2+ and Mg2+) and common food additives (BA, Glu, PS, Suc, Nat and Ino) in food were selected as research objects. The results showed that compared with H2O2, Cit-CS-UCNPs-MnO2 did not react strongly to other added substances, and the overall fluorescence signal of the sensor did not fluctuate much. Therefore, it can be seen that Cit-CS-UCNPs-MnO2 can detect the specificity of H2O2 and TBHQ.