Host-sensitized rare-earth-ion lighting has emerged as a promising route to realize single-phase white-light emission due to effectively overcoming the self-absorption problems within multiple phosphors. However, the involved energy transfer mechanism that refers to the kinetic processes of host-sensitized excitation and the complex interactions between excitons and activators are far beyond understood. Here, a single-phase white-light-emitting phosphor, Dy³⁺ doped LaInO₃ (Dy-LIO), is explored through tuning the exciton behaviors and host-sensitized emission. As phosphors are excited through the interband transiton, the LIO host presents a broadband blue emission due to the recombination of self-trapped excitons (STEs), and Dy³⁺ ions acquire the excitation energy by grabbing STEs for lighting. Dy³⁺ doping reduces the InO₆ octahedron distortion and meanwhile releases excitons, which in turn are captured by Dy³⁺ activators for energy transfer. Thus, dopant Dy³⁺ emits effective blue and yellow luminescence. In addition, the combined action of the undermined STE emission and the promoted Dy³⁺ luminescence generates a warm-white-light, which exihibits excellent thermal stability. The results reveal the exciton-mediated energy transfer processes from host (LIO) to activators (Dy³⁺) in Dy-LIO, with the energy transfer efficiency obtained as high as 77.2%. These findings build up a clear energy transfer mechanism between excitons and activators and further suggests a promising way to explore single-phase white-light-emitting phosphors.