Nitrogen dioxide (NO2) is a serious menace to human health and the environment, it is of great practical meaning to develop a fast and accurate gas sensor for monitoring low-concentration NO2 at near room temperature (RT). Here, the polymorphism of In2O3 was regulated with controllable mixed-phase ratios of hexagonal In2O3 (h-In2O3) and cubic In2O3 (c-In2O3) via a graphene oxide (GO)-mediated solvothermal approach and annealing treatment. The GO-mediation markedly promoted the formation of h-In2O3 and defect content. The morphology, specific surface area, surface chemical state and activation energy were roundly analyzed. The NO2-sensing performance results showed that the sensor based on an 8wt% GO-mediated In2O3 sample (In2O3-8) exhibited an ultrahigh response of 1021 towards 1 ppm NO2 at 30℃, which was 2.3 times higher than that of In2O3 without GO-mediation. At the same time, the response/recovery time of In2O3-8 was significantly shorter than other sensors. Additionally, the In2O3-8 sensor possessed good repeatability and long stability. In2O3-8 with an appropriate GO-mediated content provided a suitable h-In2O3/c-In2O3 phase junction interface, abundant oxygen vacancies and elevated Fermi energy levels of In2O3, which activated the surface sites and ultimately enhanced the sensing performance.