A high-efficient and thermochromic phosphor Na3Sc2P3O12: Eu2+, Mn2+ is synthesized by solid-state reaction. The phosphor exhibits blue (Eu2+) and red (Mn2+) dual-emission with high quantum efficiency of 98%. The dual-emissions has different thermal dependencies due to the competition between thermal-activated energy transfer and thermal quenching, thus enabling their intensity ratio to be highly sensitive to the temperature variation, which makes this phosphor a good candidate as ratiometric and colormetric temperature sensor. [GRAPHICS] Dual-emitting and thermochromic Eu2+, Mn2+ co-doped Na3Sc2P3O12 (NSPO) phosphor was synthesized by a modified solid-state method. Its crystal structure and luminescent properties, especially temperature-dependent fluorescence properties are discussed in detail. Na3Sc2P3O12: Eu2+, Mn2+ simultaneously exhibits two broad bands under single UV excitation, i.e. Eu2+ blue emission band centered at 460 nm and Mn2+ red emission band centered at 600 nm with quantum efficiency of > 90%. The dual emissions in Na3Sc2P3O12: Eu2+, Mn2+ have different thermal responses due to the competition between trap-to-activator energy transfer and thermal quenching, by which a novel self-referencing non-invasion optical thermometer with high sensitivity and superior signal discriminability was designed. The temperature sensing properties of Na3Sc2P3O12: Eu2+, Mn2+ in a wide range of 293-473 K were analyzed, which shows good linear relationship between ratiometric intensity and temperature (R-2 > 0.999) as well as high sensitivity (1.556% K-1). Besides, the emitting color is highly sensitive to the temperature variation. The results suggest that Na3Sc2P3O12: Eu2+, Mn2+ could be a good candidate high-sensitive for optical thermometry and safety sign in high temperature environment.