NaErF 4 is the most extensively studied host for self-sensitized upconversion (UC), and Yb 3+ is the most commonly used energy absorber. It has been reported that the red luminescence of Er 3+ can be enhanced by introducing Yb 3+ into the NaErF 4 host lattice, where Yb 3+ ions serve as trapping centers to confine the excitation energy. Also, it has been pointed out that the Yb 3+ doping in the shell of NaErF 4 -hosted core-shell nanocrystals can further improve the red emission intensity. Conversely, it can be argued that the Yb 3+ doping in the shell always results in the luminescence quenching of the NaErF 4 core. These imply that the impact of Yb 3+ on NaErF 4 -based host-sensitized UC is rather complicated and must be probed deeply. In this study, we thoroughly discussed the effect of Yb 3+ located in the core/shell on the NaErF 4 -based host sensitization UC and afforded the related mechanism interpretations. In the NaErF 4 core nanocrystals, the green-dominated UCL presented an enhancement on increasing the concentration of the Yb 3+ dopant owing to the promoted energy harvesting for luminescence. Furthermore, the emission properties of NaErF 4 :10%Yb shelled with diverse inert layers were also investigated, and the intensity difference of these core-inert shell nanoparticles could be explained by the lattice mismatch and shell thickness. In NaErF 4 :10%Yb@NaYF 4 :Yb with variable Yb 3+ doping in the shell, the red-dominated UCL was generally weakened with more Yb 3+ localized in the shell, which was ascribed to the competition of energy pooling and energy dissipation of Yb 3+ in the outer layer. Therefore, Yb 3+ ions wield a two-sided influence (termed a "double-edged sword") on the UC emissions of the Er 3+ host. Additionally, we demonstrated the application potential of such UCNPs in water sensing and high-level anti-counterfeiting. This work offers an in-depth insight into the UC mechanism of Yb 3+ -doped NaErF 4 nanocrystals and inspires the engineering of novel luminescent materials with distinguished properties.