Upconverting particle (UCP)-assisted near-infrared (NIR) photopolymerization (UCAP) is a novel photopolymerization method, in which the upconversion luminescence efficiency of UCPs is the key factor that affects the photopolymerization efficiency. In this study, a series of NaYbF₄∶Tm³⁺ microcrystals are synthesized by an ethylenediamine tetraacetic acid (EDTA)-assisted hydrothermal method, and the effects of adding sodium fluoride (NaF) and EDTA, the doping ratio of the sensitizer, and the hydrothermal temperature on the morphology and upconversion luminescence intensity of UCPs are investigated. Furthermore, the effects of UCPs on near-infrared photopolymerization are studied. The results indicate that the addition of the fluorine source promotes the phase transition and increases the aspect ratio of the crystals, and that a moderate amount of the fluorine source can enhance the upconversion luminescence of the product. In addition, decreasing the doped amount of the sensitizer, elevating the temperature, and decreasing the amount of ligand could each enhance the fluorescence intensity of UCPs to some extent. Moreover, increasing the intensity of NIR light could enhance the fluorescence intensity and the contribution of the UV emission band of the particles, resulting in a faster polymerization rate as well as a higher functional group conversion rate. Finally, the UCPs synthesized under optimal conditions could be successfully applied in direct 3D printing of NIR light-curable inks, demonstrating their application potential in 3D printing.