Mixed-valence europium-activated phosphors are receiving attention in many fields, such as lighting, anti-counterfeiting, optical recording, encryption, and temperature sensing. However, it remains difficult to construct mixed-valence europium-activated compounds due to the reductive and oxidative synthesis conditions required to obtain Eu 2+ and Eu 3+ ions, respectively. Herein, mixed-valence Eu 2+ /Eu 3+ was realized in the CaBPO 5 built by rigidly connected BO 4 and PO 4 tetrahedrons by partial Eu 3+ → Eu 2+ self-reduction in the air atmosphere. Commendably, the CaBPO 5 :Eu 2+ /Eu 3+ phosphor exhibits excellent ratiometric temperature sensing performance with the maximum absolute and relative sensitivity being as high as 0.184 K -1 and 3.444% K -1 with good signal discriminability, due to the high and low, respectively, temperature-dependence of Eu 2+ and Eu 3+ emissions. The rapid dropping intensity of Eu 2+ in CaBPO 5 with increasing temperature was due to the small energy gap (∼0.48 eV) between the Eu 2+ -5d state and the conduction band. Our work not only provides a novel thermometer candidate but also enlightens researchers to a method of effectively designing new mixed-valence metal-ion activated luminescent thermometers via selective tetrahedrally coordinated rigid crystal structure.