In order to effectively improve the electrical properties of flexible thin film transistors, HfO2 and Ta2O5 were used on polyimide (PI) substrates at room temperature. The stack structure combining two high dielectric constant materials replaces the single layer Ta2O5 as the gate dielectric to explore its effect on the device electrical properties. The thin films were prepared by magnetron sputtering, and the effects of Ta2O5 layers in the stacking gate dielectric structure on the device electrical properties under different sputtering time and different oxygen-argon ratio were investigated. The HfO2/Ta2O5 stacking gate dielectric devices were compared to Ta2O5 single-layer gate dielectric devices. The results show that when the sputtering time of Ta2O5 gate dielectric layer is 1 h and the ratio of oxygen to argon is 10∶90, the electrical performance of the device is the best. Moreover, the introduction of the stacking gate dielectric structure significantly improves the electrical performance of the device with the Ion/Ioff ratio of 1.27×106, the threshold voltage of 9.1 V, the subthreshold slope of 0.54 V/decade and the carrier mobility of 7.03 cm2/(V·s).