Due to the strong biological penetration, large stoke shift, good photostability and biocompatibility, up-conversion nanoparticles (UCNPs) were widely used in biomedical analysis. In this study, OA-UCNPs, which are UCNPs with oleic acid (OA) as ligand, were modified by polyacrylic acid (PAA)in order to prepare hydrophilic nanoparticles PAA-UCNPs, and then aptamers (Apt) were covalently coupled to the surface of PAA-UCNPs by amidation reaction to obtain Apt-UCNPs. A method for specific detection of sulfadimethoxine (SDM)was constructed by using Apt-UCNPs as energy donors and black hole quencher (BHQ1) as an energy acceptor fluorescence resonance energy transfer. The structure and performance of UCNPs, PAA and PAA-UCNPs were characterized by infrared spectroscopy (FTIR) and scanning electron microscope (SEM). The results of FTIRshowed that compared with UCNPs, PAA-UCNPs appeared at a new peak at 1 722 cm^-1 which was attributed to the -C=O stretching vibration peak of PAA, and the wide band around 3 400 cm^-1 attributed to the O—H stretching vibration of PAA; the results of the SEM test indicated that the size of UCNPs and PAA-UCNPs was 31 and 49 nm, respectively. It may be because the molecular volume of long-chain PAA is larger than that of oleic acid, so coating on the surface of UCNPs will increase its size, and the above results indicated PAA may have had been modified to the surface of UCNPs. The structure and properties of Apt-UCNPs, Apt and PAA-UCNPs, were characterized by ultraviolet-visible spectroscopy. It was found that compared with PAA-UCNPs, Apt-UCNPs had a more obvious Apt characteristic absorption peak at 260 nm, which showed that the aptamer could have been modified to the surface of UCNPs. In addition, the mechanism of using Apt-UCNPs to detect SDM was also discussed. The results showed that the emission peak of Apt-UCNPs at 540 nm overlapped with the absorption peak of BHQ1, which indicated that the energy on Apt-UCNPs can be transferred to BHQ1 through the fluorescence resonance energy transfer effect so that the fluorescence of Apt-UCNPs could be quenched. The concentration of BHQ1 in the detection system was optimized and the results showed that when the concentration of BHQ1 was 15 μmol·L^-1, the fluorescence quenching efficiency was as high as 55%. Under the best experimental conditions, there was a good linear relationship between the relative fluorescence intensity and the SDM concentration (150~1 000 ng·mL^-1).The analogues of SDM (sulfapyridine and sulfacetamide)were selected as control experiments. It was found that although the concentration of sulfapyridine and sulfacetamide reached 500 ng·mL^-1, the fluorescence intensity recovery in the system was less, which indicated that the detection method could specifically recognize SDM.