Lanthanide-doped Upconversion Luminescent (UCL) materials have attracted significant attention due to their distinctive optical properties, including excellent photostability, large anti-stokes shifts, high signal-to-noise ratios, and long emission lifetimes. These characteristics make UCL materials highly suitable for a wide range of applications, such as white light-emitting diodes, solid-state laser, and liquid crystal displays. Recently, white light emission from lanthanide-doped UCL materials has exhibited significant potential for applications in three-dimensional backlight technology. Upconversion white light is generally achieved by fine-tuning the emission intensities of red, green, and blue light. The red and green emission can be effectively modulated by varying the concentrations of Er3⁺ or Ho³⁺ ions, while the blue emission intensity is directly influenced by the concentration of Tm3⁺ ions. However, the effective regulation of the UCL spectrum and a deep understanding of its emission mechanisms remain significant scientific issues. Most researches on UCL have focused on the optical properties of micro/nanoparticle ensembles, where the collected spectra reflect an averaged emission effect from a large number of luminescent particles. Due to interparticle interactions, these measurements cannot accurately represent the emission behavior of individual crystal particles, making it even more challenging to uncover the fundamental photophysical mechanisms at the single-particle level. Therefore, exploring the luminescent properties of single lanthanide-doped upconversion crystal is crucial for the development of new materials and the optimization of their performance. In this work, we fabricate β-NaYF₄:20% Yb³⁺/x%Er³⁺/0.2%Tm³⁺ (x=0.2,0.4,0.6,0.8,1.0) microcrystals the uniform morphology and size, featuring a natural hexagonal cross-section and wedge-shaped on both top vertexes. The microcrystals have an average length approximately 40.2 μm. Based on a selt-built confocal microscopy system, the upconversion luminescence performance of a single β-NaYF₄:20% Yb³⁺/x%Er³⁺/0.2%Tm³⁺ (x=0.2,0.4,0.6,0.8,1.0) microparticle is detected and studied. A gradual shift in the UCL color of a single β-NaYF₄ microcrystal from yellow-green to white was observed as the doping concentrations of Er³⁺ ions increased under 980 nm laser excitation. The upconversion luminescence emission of individual microcrystal primarily originates from the transitions of Er3? ions: ²H_11/2/⁴S_3/2→⁴I_15/2 (520 nm and 539 nm, green) and ⁴F_9/2→⁴I_15/2 (653 nm, red), and the transitions of Tm3? ions: ¹D₂→³F₄ and ¹G₄→³H₆ (450 nm and 475 nm, blue), ¹G₄→³F₄ and ³F₃→³H₆ (650 nm and 696 nm, red). In β-NaYF?:20% Yb3?/x% Er3?/0.2% Tm3? (x=0.2,0.4,0.6,0.8,1.0) microcrystals, the intensities of red and green emissions increase with the increasing Er3? ion concentration, while the blue emission intensity decreases rapidly with higher Er3? ion concentration. Ultimately, white light UC emission was successfully realized mainly due to competitive energy transfer between Yb³⁺→Er³⁺ and Yb³⁺→Tm³⁺, as well as cross-relaxation process between Er³⁺ and Tm³⁺ ions. Furthermore, the UCL polarization characteristics of a single β-NaYF₄:20%Yb³⁺/0.2%Er³⁺/0.2%Tm³⁺ microcrystal were systematically investigated under the linearly polarized light excitation. As the polarization angle varies from 0° to 360°, the upconversion luminescence intensity of a single β-NaYF₄:20%Yb³⁺/0.2%Er³⁺/0.2%Tm³⁺ microcrystal exhibits periodic changes. When the laser polarization direction aligns parallel or perpendicular to the long axis of the microcrystal, the upconversion luminescence intensity of the single microcrystal reaches maximum and minimum, respectively. Additionally, by tuning the excitation light focused on different positions of a single microcrystal, the UC white light emission intensity exhibited obvious periodic variations as the excitation polarization direction and the angle between the microcrystal’s wedge-shaped end and the slide were adjusted. These results demonstrate the strong dependence of the UCL intensity on the polarization of excitation light in the single β-NaYF₄:20%Yb³⁺/0.2%Er³⁺/0.2%Tm³⁺ microcrystal. This research provides valuable insights into the spatial orientation and luminescence mechanisms of single anisotropic upconversion micro/nanocrystals and holds significant implications for advancing applications in anti-counterfeiting technologies and efficient fluorescent probes for bioimaging.