In the preparation processes of aluminum oxynitride (AlON) powders by carbothermal reduction and nitridation, the homogeneity of mixed raw powders between Al₂O₃ and C is a critical factor by which the final composition and related properties of AlON transparent ceramic will be decided. In this paper, a silane coupling agent was used as a dispersant to optimize the distribution uniformity of raw material of Al₂O₃ and C, and the preparation of AlON powder with controllable composition and its distribution is investigated. The results show that the silane dispersant could effectively improve the distribution uniformity of raw material. The silane coupling agent contains functional groups of −SiH₃ and −C_nH_2n+1O. XPS showed that the silane could react with C and Al₂O₃ to form the Si–C bond and C–Al₂O₃ bond, respectively. The silane coupling agent provides a connected bridge for raw material powders. When the amount of the silane was 5 wt%, the mixed powder had a great distribution uniformity. The addition of silane coupling agent improved the reactivity of raw materials and decreased the synthesis temperature of AlON. The single-phase AlON powder was obtained after the Al₂O₃/C mixed powder was kept at 1670 °C for 30 min. Furthermore, the grain size of AlON powder was 100–200 nm with an AlN content of 27.5 mol%. With the increase of holding time to 4 h, the grain size increased to 15 μm, indicating that sintering between particles occurred, which may reduce the sintering activity of the powder.