In　spite　of　decades　of　great　efforts,　it　is　still　very　challenging　to　realize　accurate　design　and　synthesis　of　secondorder　nonlinear　optical　(NLO)　crystals.　KBe2BO3F2　(KBBF)　is　the　sole　NLO　material　that　can　output　deepultraviolet　light　by　direct　second-harmonic　generation　process,　so　it　is　an　ideal　choice　to　use　KBBF　as　the　structural　template　to　design　environment-friendly　candidates　with　better　growth　habit　based　on　the　reported　compounds.　In　this　review,　we　will　elaborate　the　accurate　design　and　synthesis　of　high-performance　NLO　materials　from　the　perspective　of　microscopic　crystal　structure　based　on　KBBF,　hoping　to　provide　deep　understanding　on　how　to　rationally　design　and　synthesize　targeted　compounds　efficiently　and　quickly.　In　addition,　according　to　multiple　requirements　on　ultraviolet　NLO　crystals,　including　second-harmonic　generation,　cut-off　edge,　and　birefringence,　we　propose　a　new　concept　so-called　quality　factor　to　comprehensively　evaluate　the　overall　performance　of　new　ultraviolet　NLO　materials.　This　concept　would　help　to　directly　and　readily　evaluate　the　ability　of　an　ultraviolet　NLO　material　to　achieve　practical　NLO　output.