In order to improve the stability of polymer thin-film transistors（PTFTs）,PTFTs based on poly（3-hexyl-thiophene）（P3HT） thin films were fabricated on silicon substrate,with SiO2 as the gate insulator and with gold as the source and drain electrodes.Then,the electrical characteristics and air-stability of the devices were investigated,and the mechanism of the instability in air was discussed.The results indicate that,with the increase of exposure time,both the saturation current and the threshold voltage of the devices exposed to ambient air increase ob-viously,that the moisture of ambient air is the key to the device characteristics,and that the passivation treatment with a photoresist layer can effectively improve the stability of the devices exposed to ambient air and increase the carrier mobility by about 3 times.