In order to improve the electrical performance of oxide thin film transistors, In-doped ZnO thin film transistors (IZO TFT) were prepared by using a sputtered IZO thin film as the active layer and an Al₂O₃ thin film deposited by atomic layer deposition (ALD) as the gate dielectric layer. The effects of the ratio of argon and oxygen, sputtering gas pressure, and annealing temperature on the electrical properties of IZO TFT were investigated during the IZO film preparation process. The results indicated that the IZO TFT, which was prepared at the appropriate argon-oxygen ratio and reaction pressure under relatively high annealing temperatures, shows excellent electrical characteristics. When the argon oxygen flow rate ratio was 60:20 sccm, the sputtering gas pressure was 0.5 Pa, and the air annealing temperature and time were 250 ℃ and 1 hour, respectively. The electrical properties of the IZO TFT were relatively better with a carrier saturation mobility of 31 cm²/(V·s) and a high on-off current ratio of 10⁸. A relatively too-low or too-high argon-oxygen ratio could cause too-low or too-high oxygen vacancies in the active layer of IZO, thus reducing TFT device performance. The low annealing temperature couldn't transform the Al-OH bonds in the gate dielectric layer into Al-O bonds, and was difficult for oxygen in the air to diffuse into IZO and passivate oxygen vacancies, thus leading to poor device performance.