We have fabricated Al-doped ZnO thin-film transistors (AZO TFT) on plastic substrate at room temperature. The effect of O2/Ar ratio during channel deposition is investigated on the electrical properties of the Al-doped ZnO thin-film transistors (TFTs) on plastic substrates at room temperature. As the O2/Ar ratio increases, the threshold voltage increases monotonously while the saturation mobility and the subthreshold swing show a non-monotonic change, reaching a maximum and a minimum respectively when the ratio of O2 is 5%. And it's further demonstrated that TFTs with as-deposited AZO films (5%O2) exhibit the best electrical properties, with an on/off current ratio of 106, an of current in the order of 10-11 A, a threshold voltage about 1.06 V, a subthreshold swing of about 0.76 V/dec, and a saturation mobility of about 47.76 cm2V-1 s-1. Compared to the AZO film with 100% Ar during the radio-frequency (RF) sputtering deposition, the grain size of AZO films abruptly decreases to a few nanometers as O2 is added(5% O2), and then becomes almost unchanged with the further increase of pO2 (10% O2 and 20% O2). The resistivity experiences a substantial increase correspondingly The results show that Al-doped ZnO is a promising candidate for the channel material of high performance flexible TFTs.