All-solid-state deep ultraviolet (DUV) coherent light sources (λ<200 nm) have important applications in frontier science, high technology and many other fields. An effective and feasible technical approach is to use commercially available visible and near-infrared all-solid-state lasers as the fundamental frequency light source to generate DUV laser through cascaded frequency conversion by nonlinear optical (NLO) crystals. The basic conditions of DUV NLO crystals are described from two aspects of birefringence and dispersion, and the research progress of DUV NLO crystals and technical difficulties need to be overcome for applications are introduced. Taking KBe2BO3F2 (KBBF) and RbBe2BO3F2 (RBBF) as examples, the discovery, crystal growth, the corresponding prism-coupled device technology, their main optical properties, and their ability to generate DUV coherent light are reviewed. The research progress of DUV coherent light sources based on KBBF crystals and prism-coupled technology are discussed in detail, as well as their applications on the advanced scientific instruments, especially on ultra-high-resolution photoelectron spectrometers. At last, the perspectives for future development of DUV NLO crystals and all-solid-state DUV laser technology are given.