Device stability is one of the key parameters for transistor applications. To improve the stability of Indium oxide (In2O3) after a long-time gate bias, a synthetic solution of hafnium chloride (HfCl4) and indium nitrate (In(NO3)3∙xH2O) reagents were used to obtain 0% to 5-at.% Hf doped In2O3 thin-film transistors. With the increase of Hf doping concentration, oxygen vacancies and residual hydroxyl groups continue to decrease, suppressing the carrier concentration and influencing the trap state density of In2O3. The sub-threshold slope (SS) 0.78 V·dec−1 for the undoped In2O3 transistor in this work is a typical value. When the dopant dose is up to 5-at.%, SS decreases to 0.32 V·dec−1. According to the proportional relationship between SS and the density of trap states, it shows that the density of trap states in the dielectric layer and the semiconductor/dielectric interface SS is greatly reduced after 5-at.% Hf doping. The probability of the charge being trapped is dropped as well. At the same time, under the doping of Hf, the transistor exhibits a very small threshold voltage shift. Especially at the dopant dose of 5-at.%, the transfer characteristic curve hardly shifts. This work demonstrates an In2O3 transistor with high bias stability by doping method.