Noncontact optical thermometers have attracted widespread attention, but existing problems such as single-mode and low-sensitivity thermometers still urgently need to be solved. Herein, a novel multiple-mode thermometer was designed for the polymorphism LaSc₃(BO₃)₄:Eu^2+/3+,Li⁺. X-ray diffraction (XRD) patterns revealed a slight transition between α- and β-phases with the concentrations of the dopants, which is further proved by structure refinements and first-principles calculations. The coexistence of Eu²⁺ and Eu³⁺ in the phosphors and their relative percentages were confirmed by X-ray absorption near-edge structure (XANES) spectra. Benefiting from appropriate emissions from Eu²⁺ and Eu³⁺ without obvious energy transfer and their opposite changing trends with temperatures under 307 nm excitation, a triple-mode optical thermometer is obtained for this material within the temperature range of 150–450 K. The highest sensitivities of 27.65, 14.05, and 7.68 %·K⁻¹ are achieved based on two fluorescence intensity ratio (FIR) modes of Eu²⁺ and Eu³⁺ (5d–4f/⁵D₀–⁷F_2,4) and the fluorescence lifetime (FL) mode of Eu²⁺, respectively. To the best of our knowledge, the former is almost the highest in Eu²⁺ and Eu³⁺ co-doped thermometers. These results indicate that this material may be used as an excellent multiple-mode optical thermometer.