In this paper, we demonstrated the synthesis of Li_1−xNa_xYF₄:Yb³⁺/Ln³⁺ (0 ≤ x ≤ 0.3, Ln = Er, Tm, Ho) upconversion nanocrystals (UCNCs) via a simple high boiling solvent process. The size and morphology of the Li_1−xNa_xYF₄ NCs can be tuned by varying the doping amount of Na⁺. Furthermore, the introduction of Na⁺ also affects their upconversion luminescence properties. By changing the dopant's species (Ln³⁺) and the doping concentration of Na⁺, multicolor (green, blue, red, yellow, orange and white) upconversion emissions can be obtained. In addition, the as-prepared OA-capped hydrophobic LiYF₄:Yb³⁺/Er³⁺ nanocrystals were modified with sodium dodecylsulfate (SDS) to convert them into hydrophilic ones. MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) assay indicates that the SDS-modified LiYF₄:Yb³⁺/Er³⁺ NCs exhibit good biocompatibility. Meanwhile, the application of these hydrophilic LiYF₄:20%Yb³⁺/2%Er³⁺ and Li_0.7Na_0.3YF₄:20%Yb³⁺/2%Er³⁺ NCs in the luminescence imaging of cells were also investigated, which showed bright green and red emissions without background noise, respectively. So these UCNCs are promising for multiple cell imaging.