<p>Rare earth-doped upconversion nanomaterials are one of the luminescent materials that have received extensive attention in recent years and have important applications in fields such as biomedicine, optoelectronic devices, and security inspection. In this study, for NaYF₄:Yb³⁺,Er³⁺ upconversion nanoparticles, the effects of experimental parameters such as reaction temperature, reaction time, and the content of surfactants on their crystal phase, size, and morphology were systematically investigated. By adjusting the experimental parameters, the size of the hexagonal phase upconversion nanoparticles can be effectively controlled within the range of 18–71&#xa0;nm. The study found that as the size of the nanoparticles increases, the upconversion luminescence intensity gradually increases. With the help of the first-principles theoretical calculations, an ion distribution model of upconversion nanoparticles varying with size was successfully constructed. Based on the model, the experimental scheme was optimized, and upconversion nanoparticles (UCNPs) with a size of 70.82&#xa0;nm were prepared successfully. The luminescence intensity of the UCNPs was increased for about 6 times. Based on the above research, a near-infrared invisible dynamic response anti-counterfeiting label was prepared, and the optical response effect of the near-infrared detector based on upconversion nanoparticles was demonstrated. This work provides an important reference for the application of upconversion nanomaterials.</p>