Aluminum oxide (Al2O3) thin films are prepared by radio frequency magnetron sputtering at room temperature, and the optimal control of the properties of these films is realized by adjusting the sputtering pressure. The prepared films with the best thickness uniformity are obtained under the sputtering power of 120 W and Ar pressure of 0.13 Pa, whose ratio of aluminum atoms to oxygen atoms is 1∶1.67, density is 3.21 g/cm3, and surface roughness is 0.62 nm. This smooth and compact Al2O3 thin film can significantly reduce the defects, and it also possesses the characteristics of high breakdown voltage, high relative dielectric constant and low leakage current. The flexible α-IGZO-TFTs are prepared on a polyimide (PI) substrate at room temperature by utilizing the optimized Al2O3 films as the gate insulation layer, and exhibit electrical performance with a mobility of 2.19 cm2/(V·s), an on/off current ratio of 105, a sub-threshold swing of 0.366 V/decade, and a threshold voltage of 3.01 V.