Amorphous InGaZnO thin-film transistors （a-IGZO TFTs） passivated by a polyimide （PI） layer has been fabricated. The PI passivation layer has been deposited by the spin-coating method， which could reduce the damage of back surface of a-IGZO channel caused by the passivation layer deposition process and improve the device performance . The PI passivation could effectively block the ambient atmosphere， protecting the TFTs from performance deterioration resulted by H₂O and O₂ molecule adsorption effects. Moreover， TFTs passivated by PI showed improved stability under negative gate-bias stress （NBS）. The improved stability was inferred to be related to the passivation of traps within the a-IGZO channel by hydrogen which could be diffused into the device from the PI layer during the baking process.