Lanthanide ion‐based upconversion is a highly promising approach for achieving deep‐ultraviolet (DUV) emission. In particular, the visible‐to‐ultraviolet upconversion process of Pr³⁺ ions, which proceeds via an intermediate energy level, serves as an efficient technical approach to realize high‐efficiency DUV emission under easily accessible blue light excitation. Herein, through 5% Lu³⁺ doping, we have strengthened the lattice rigidity of the tetragonal yttrium oxybromide (YOBr) host, resulting in an approximately 40% enhancement in the DUV luminescence performance of Pr³⁺. Mechanistic studies show that the enhancement is attributed to the highly suppressed non‐radiative relaxation from the 4f¹5d¹ state without significant thermal ionization. Moreover, we unambiguously demonstrate the formation of broadband DUV random lasing, Fabry‐Pérot cavity lasing, and whispering‐gallery‐mode microcavity …