Amorphous indium gallium zinc oxide thin film transistors (a-IGZO TFTs) have the advantages of high field effect mobility and high switching ratio, and thus exhibit great potential applications in the fields of active drive display technology, flexible and wearable electronic devices. However, the poor stability of the electronic performances is the key problem restricting its large-scale applications. In this paper, Al2O3 thin films with low oxygen vacancy content were prepared by hollow cathode assistant pulsed laser deposition at room temperature, and used as the passivation layer of a-IGZO TFT devices. In-depth X-ray photoelectron spectroscopy illustrates that oxygen plasma introduced by the hollow cathode inhibits the formation of oxygen vacancies at the Al2O3/a-IGZO interface, which could improve the subthreshold performances of the TFT devices. 180 oC annealing for a-IGZO films could improve the mobility and reduce the threshold voltage shift; Annealing at 100 oC for Al2O3/a-IGZO TFT devices is helpful to reduce the carrier concentration and improve the subthreshold characteristics. The mobility of TFT devices processed by these two combining annealing processes could be as high as 22.8 cm2V-1s-1, and the subthreshold swing is 0.6 Vdecade-1.