We have developed a simple and versatile strategy for in situ growth of MnO2 on the surfaces of oleic acid-capped hydrophobic upconversion nanoparticles (UCNPs) by optimizing the component concentrations in the Lemieux-von Rudloff reagent. The oxidation time was shortened by a factor of two compared to that of the reported method. This oxidation process has no obvious adverse effects on the phases of UCNPs. STEM, X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FTIR) and energy-dispersive X-ray analysis (EDX) characterization demonstrated the successful growth of MnO2 on the surfaces of UCNPs. Furthermore, when the weight ratio of MnO2/UCNPs reached (147.61 ± 17.63) μg mg-1, 50% of the initial upconversion luminescence of UCNPs was quenched, as revealed by fluorescence and inductively coupled plasma optical emission spectrometry (ICP-OES) results. The presence of the surface MnO2 precipitate not only confers high dispersity of UCNPs in water, but also allows further activatable magnetic resonance imaging (MRI) and fluorescence multimodal imaging after reduction to Mn2+ by intracellular glutathione (GSH). A novel targeted drug carrier nanosystem was prepared to protect MnO2 from early decomposition in blood circulation by coating with mesoporous silica and capping with a gelatin nanolayer. Aptamer sgc8 was then attached to the surface of the gelatin nanolayer by covalent crosslinking to achieve targeted drug delivery. The results suggest that this nanosystem shows promise for further applications in cancer cell imaging and therapy.