Rare-earth nanocrystals are a family of important nanomaterials due to their unique optical and electronic properties, and they are paid more attention for their applications in the areas of solar energy system, molecular imaging, optoelectronics and catalysis fields. The incorporation of rare-earth nanocrystals into smart gels, which combines the PL features of rare-earth nanocrystals with the reversible and stimuli-responsive properties of smart gels, will provide a new generation of fluorescence markers for practical application. In this paper, we report a one-pot method to prepare thermosensitive fluorescent nanogels from rare earth nanocrystals and PNIPAm-co-PAA nanogels. Firstly, using cysteamine, Er(NO3)3, Yb(NO3)3, Y(NO3)3 and NH4F as raw materials, active YF3:Yb3+-Er3+ nanocrystals with-NH2 groups were synthesized by hydrothermal method. After that, in the present of the as-prepared active YF3:Yb3+-Er3+ nanocrystals, N-isopropyl acrylamide (NIPAm) and N,N'-methylene bisacrylamide (BIS) were initiated by K2S2O8.Finally, thermosensitive fluorescent nanogels of YF3:Yb3+-Er3+/PNIPAm-co-PAA were fabricated by free radical polymerization and in-situ coupling reactions of 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC) and N-hydroxysuccinimide (NHS). The microstructure and performance of the as-prepared YF3:Yb3+-Er3+ nanocrystals and the complex nanogels were investigated by differential scanning calorimeter (DSC), photoluminescence (PL), Fourier transform infrared (FTIR), X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM). As the evidence from the HRTEM images, active YF3:Yb3+-Er3+ nanocrystals show well monodispersity with their size of about 6～10 nm, and the complex nanogels are polydisperse ones with their size of about 100～300 nm. The results of PL spectra at various temperatures suggest that, the active YF3: Yb3+-Er3+ nanocrystal present weak double emitting peaks around 483 and 496 nm, which are resulted from the energy level splitting of 4F7/2→4I15/2 transition of Er3+. As for the complex nanogels, the phenomenon of energy level splitting of Er3+ is different from that of the nanocrystals, with increasing ambient temperatures, double emitting peaks around 483 and 496 nm couple into an emitting peak around 489 nm gradually, and their intensity decreases correspondingly.