Lead-free metal halide perovskites have become emerging candidates for white light emitting diodes and X-ray imaging, and exploration of the corresponding down-conversion phosphors and scintillators based on perovskite structures with a high-performance for advanced devices is ongoing. Here, we synthesized Cs2HfCl6: Te4+ crystals with a yellow emission at 557 nm and a wide full width at half maximum around 104 nm, which is attributed to the transition from the triplet state to the ground state (P-3(1) -> S-1(0)) of Te4+ ions. Moreover, the energy transfer process from the self-trapped exciton of Cs2HfCl6 to Te4+ ions is demonstrated. Accordingly, the white light emitting diodes show a color rendering index of 61.8, which was manufactured by employing the Cs2HfCl6:3% Te4+ crystals with a photoluminescence quantum yield of 74.2%. Furthermore, the Cs2HfCl6: Te4+ crystals exhibit a fascinating steady-state X-ray light yield reaching 76,194 photons MeV-1, a low detection limit of 36.2 nGy s(-1) and a high spatial resolution of 11 lp mm(-1). Hence, this study offers new insight for the development of lead-free metal halide perovskites and promotes their versatile application in white light emitting diodes and X-ray imaging.