By employing ultra-fast high-temperature sintering (UHS) followed by conventional sintering (CS), aluminum oxynitride (AlON) ceramics with high transmittance (≥ 80%) over a wide wavelength range of 350–4900 nm were successfully fabricated by holding for 2 min at 1850 °C for UHS and subsequently 120 min at 1880 °C for CS. The relative density, pore size, and grain size were compared among the samples prepared by UHS, UHS+CS, and CS to investigate the mechanism behind the excellent transparency achieved through UHS+CS. After UHS, AlON with a high relative density of 97.43% was obtained, which was much greater than the 88.51% of the sample prepared at 1850 °C via CS. Furthermore, the CS of the UHSed AlON increased its relative density to 99.71%, with fewer small pores (≤ 0.37 µm) observed on the fracture surface. These properties are noticeably better than those of the sample directly sintered with CS (relative density of 99.30%, pore size of 0.45–2.40 µm). The high relative density of the UHSed sample, along with the predominance of small grains and nonequilibrium grain boundaries induced by UHS, contributes to the rapid densification of AlON during subsequent CS. This results in the excellent transparency and high hardness (HV = 19.57±0.23 GPa) of the final product. Consequently, UHS+CS is an effective and efficient strategy for fabricating highly transparent AlON ceramics, with UHS being a crucial step.