The optical transparent region is a crucial parameter for optoelectrical functional crystals. Herein, the relationship between the infrared (IR) absorption edge and phonon frequencies in inorganic optical crystals is investigated. It is demonstrated that the two-phonon absorption mechanism plays a significant role in determining the IR absorption edge in millimeter- and centimeter-level crystals, which are commonly employed for practical applications, and their quantitative correspondence is built based on first-principles calculations. Moreover, the phonon forbidden gap is highlighted to expand the transparent range. Accordingly, we predict two optical crystals with an ultrawide transparent region, i.e., Ag