New Al2O3 monolayers were predicted using density functional theory, and their stability, geometric structures, and mechanical, electronic, and optical properties were investigated. Among them, Te-Al2O3 with Pmmb space group was the most stable structure with markedly mechanical anisotropy. At the HSE06 level, Te-Al2O3 exhibited an ultra-wide indirect bandgap of 6.338 eV, which was verified to have a relatively reliable value by contrastive analysis on experiments. An exciton with a large binding energy of 1.356 eV was revealed in Te-Al2O3 by solving Bethe–Salpeter equation, and such large binding energy intensely affected the optical properties. As a result, the anisotropic absorption spectra were located at ~4.0–16 eV, nearly covering the spectrum ranging from the near- ultraviolet (UV) to far-UV regions. In particular, the first optical absorption peak appearing at ~5.0 eV indicated that Te-Al2O3 is suitable for solar-blind photodetectors. This work revealed that the ultra-wide bandgap semiconductor Te-Al2O3 has a great application in UV photodetection.