A high-performance ZnSnO (ZTO) thin-film transistor (TFT) was fabricated, with ZTO deposited by rf magnetron sputtering. XPS was used to analyze and study the effects of different annealing temperatures on the element composition and valence state of ZTO films. Then, the influence mechanism of annealing treatment on the electrical properties of ZTO thin films was analyzed. The results show that, with an increase in annealing temperature, the amount of metal bonding with oxygen increases first and then decreases, while the oxygen vacancy decreases first and then increases. Further analysis on the ratio of Sn2+ is presented. Electrical results show that the TFT annealed at 600 °C exhibits the best performance. It exhibits high saturation mobilities (μSAT) up to 12.64 cm2V−1s−1, a threshold voltage (VTH) of −6.61 V, a large on/off current ratio (Ion/Ioff) of 1.87 × 109, and an excellent subthreshold swing (SS) of 0.79 V/Decade.