Most of the long persistent luminescence (LPL) materials are investigated at room temperature or higher temperatures. However, LPL materials have promising applications in extremely low-temperature conditions. Here, we report a phosphor, namely BaGa₂O₄:Bi³⁺, with LPL that can act as a biological-temperature indicator and thermometer at extremely low temperatures. The traps were investigated in detail with the lowest and highest trap densities of 2.39 × 10⁶ and 7.23 × 10⁷, respectively. The trap distribution ranges from 150 to 750 K in the thermoluminescence (TL) spectrum with corresponding trap depths ranging from 0.310 to 0.716 eV. The TL spectra show that the distribution of the carriers stored in the traps to two emission centers changes with the temperature rising. The low-temperature steady-state photoluminescence (PL) was also investigated. Since the CIE colour coordinates are almost unchanged and the integrated emission intensity at 300 K was still maintained at 88.2% of 7 K, the phosphor presents a stable colour and intensity output. By decomposing the PL spectra at different temperatures, we noticed that one of the deconvoluted full width at half-maximum (FWHM) bands is temperature-sensitive; the thermometer-related parameters were calculated with the maximum values of S_a and S_r to be 0.163 and 0.108%, respectively. Our concept of investigation may promote the follow-up exploration of LPL phosphors under extreme conditions.