A novel Eu²⁺-Eu³⁺ co-activated ratiometric thermo-sensitive phosphor was developed and synthesized by solid-state reaction. The valence state of Eu, photoluminescence and thermo-sensitive performance of the phosphor prepared either in ambient air or carbothermally were investigated and discussed. The phosphor shows high sensitivity (S_a = 0.0173 K^–1, S_r = 0.461%/K) and superior signal discriminability (Δν = ∼10380 cm⁻¹). The thermo-sensitive performance is subject to the dual effects of different thermo-responses by Eu²⁺ versus Eu³⁺ combined with energy transfer from Eu²⁺ to Eu³⁺, so that the sensitivity of the phosphor in the temperature range presents a non-monotonic trend. The development of the BaAl₂B₂O₇:Eu²⁺,Eu³⁺ phosphor is not only expected to be relevant for application in the field of temperature sensing, but also of reference significance for improving the sensitivity by means of energy transfer between co-activator ions over a wider temperature range of Eu²⁺-Eu³⁺ co-activated ratiometric thermo-sensitive phosphors.