A series of the hexagonal-phase NaLuF ₄:20.0%Yb ³⁺/2.0%Ho ³⁺/12.0% Ce ³⁺@NaLuF ₄: x%Yb ³⁺core-shell (CS) nanocrystals with codoping different Yb ³⁺ions in the shell is successfully built by a sequential growth process. The crystal structures and morphologies of samples are characterized by X-ray diffractometer and transmission electron microscope. With the Yb ³⁺ion concentration increasing from 0% to 15% in NaLuF ₄shell, none of the crystal structures, sizes, and morphologies of the samples changes obviously because of the similarity in ionic radius between Yb ³⁺and the ions in shell and the low doping concentration. Under 980 nm near-infrared (NIR) excitation, the NaLuF ₄:20.0%Yb ³⁺/2.0%Ho ³⁺/12.0%Ce ³⁺core nanocrystal produce green and red UC emission. And the red UC emission intensity is higher than green emission intensity. This is because two effective cross-relaxation processes happen between Ho ³⁺and Ce ³⁺ions, which results in the enhancement of the red emission. However, the overall emission intensity of NaLuF ₄:20.0%Yb ³⁺/2.0%Ho ³⁺/12.0%Ce ³⁺nanocrystal decrease compared with that of the NaLuF ₄:20.0%Yb ³⁺/2.0%Ho ³⁺nanocrystal. Thus, to further enhance the red UC emission intensity in NaLuF ₄:20.0%Yb ³⁺/2.0%Ho ³⁺/12.0%Ce ³⁺nanocrystal, the NaLuF ₄:20.0%Yb ³⁺/2.0% Ho ³⁺/12.0%Ce ³⁺@NaLuF ₄: x%Yb ³⁺CS nanocrystal are prepared for blocking the excitation and emission energy, transmitting surface quenching center and getting more excitation energy through doping Yb ³⁺ions in NaLuF ₄shell. It can be clearly seen that the red UC emission intensity of CS nanocrystal first increases and then decreases with Yb ³⁺ion concentration increasing. Meanwhile, the corresponding red-to-green ratio increases from 4.9 to 5.6. The highest red UC emission intensity is observed in each of the NaLuF ₄:20.0%Yb ³⁺/2.0%Ho ³⁺/12.0%Ce ³⁺@NaLuF ₄:10%Yb ³⁺CS nanocrystal because the Ho ³⁺ions get more energy through the following three ways: 1) Yb ³⁺(core)-Ho ³⁺(core); 2) Yb ³⁺(shell)-Ho ³⁺(core); 3) Yb ³⁺(shell)-Yb ³⁺(core)-Ho ³⁺(core). Thus, building CS nanocrystals is one of the most effective approaches in order to improve the UC efficiency by suppressing the non-radiative decay of activators in the core and getting more excitation energy through different energy transfer ways. These NaLuF ₄:20.0%Yb ³⁺/2.0%Ho ³⁺/12.0%Ce ³⁺@NaLuF ₄:Yb ³⁺CS nanocrystals with red UC emission have great potential applications in biological field and multi-primary color.