Indium oxide has been demonstrated to be a suitable catalytic material for CO2 adsorption and activation, and in this study a phase-mixing strategy is used to improve its catalytic hydrogenation of CO2 to CH3OH. Mixed-phase indium oxide with controllable cubic and hexagonal phases is synthesized by a solvothermal method, and its CO2 conversion and CH3OH space-time yield are about 2 times higher than those of single-phase indium oxide. Experimental investigation shows a significant mixed-crystal effect resulting from the phase mixing, which substantially promotes oxygen vacancy formation and medium-strength CO2 adsorption and thereby enhances the catalytic performance. Furthermore, the mixed-phase catalyst is very stable and too difficult to reduce during reaction. These results give a good technique for the development of highly active and long-lived CO2 hydrogenation catalysts through crystal phase engineering.