This study reports a series of closed-loop molecules with narrowband phosphorescence through the multiple resonance effect, significantly improving the color purity of room-temperature phosphorescence (RTP). The phosphors exhibit narrowband phosphorescence with a full width at half maxima (FWHM) of 30 nm after doping into a rigid benzophenone matrix under ambient conditions, achieving an RTP efficiency of 51.8%. At 77 K, the FWHM of phosphorescence is reduced to only 11 nm. The color of the narrowband RTP can be tuned from sky blue to green by modifying methyl groups. The potential applications in X-ray imaging and display are demonstrated, highlighting the design principle for developing narrowband RTP materials and extending their potential in optoelectronics. The findings provide a foundation for advanced techniques such as ultrahigh-definition displays and high-resolution imaging.This study reports a series of closed-loop molecules with narrowband phosphorescence through the multiple resonance effect, significantly improving the color purity of room-temperature phosphorescence (RTP). The phosphors exhibit narrowband phosphorescence with a full width at half maxima (FWHM) of 30 nm after doping into a rigid benzophenone matrix under ambient conditions, achieving an RTP efficiency of 51.8%. At 77 K, the FWHM of phosphorescence is reduced to only 11 nm. The color of the narrowband RTP can be tuned from sky blue to green by modifying methyl groups. The potential applications in X-ray imaging and display are demonstrated, highlighting the design principle for developing narrowband RTP materials and extending their potential in optoelectronics. The findings provide a foundation for advanced techniques such as ultrahigh-definition displays and high-resolution imaging.