In this study, a series of NaYF₄:Yb³⁺, Tm³⁺@NaYF₄:Yb³⁺, Er³⁺ core@shell nanocrystals with different sensitizer contents in the shell were synthesized by a facile solvothermal method to investigate the impact of sensitizer Yb³⁺ on the upconversion luminescence. Transmission electron microscopy (TEM) results show that all prepared nanocrystals are well-crystallized with a hexagonal phase and have a desirable size distribution. The particle size increases after shell coating as shown in TEM images; however, it changes a little for samples with different sensitizer contents in the shell. Therefore, the effect of different particle sizes on luminescence properties, which is caused by different Yb³⁺ contents, could be minimized. Subsequently, it was assumed that the luminescent center content (Er³⁺) is same in all samples and does not influence the emission spectrum differences. Therefore, we investigated the underlying upconversion luminescence mechanism by determining the sample’s fluorescence spectrum and assuming that the energy transfer mechanism occurred in all the samples. The results show that the green upconversion luminescence decay was faster than its red counterpart when the Yb³⁺ content was increased and samples were excited using a 980 nm pulse laser. The transient spectra measured at different times after the excitation pulse reveal the intensity ratio associated with the green to red upconversion luminescence. It was concluded that the back energy transfer (BET) process from Er³⁺ to Yb³⁺ led to a conversion of the green emissions into the red emissions, which was beneficial to the red upconversion and the electron relaxation from ⁴I_11/2 to ⁴I_13/2. Therefore, the red to green upconversion luminescence ratio increases significantly in samples highly doped with Yb³⁺.