The invention provides an annulus protection fluid for a CO2 flooding injection well and a preparation method thereof. The annulus protection fluid comprises a corrosion inhibitor, a density regulator, a sterilizing agent, a pH regulator, alcohol and water. The annulus protection fluid provided by the invention has good synergy in components, so the problem of corrosion of CO2 injection well annulus can be controlled; and the annulus protection fluid has the advantages of large density regulation range, good sterilization effect, excellent pH value control effect, capability of well inhibitingcorrosion of an oil sleeve pipe, low environmentally-friendly preparation cost, low freezing point and the like, and is simple, convenient and easily-operable in on-site preparation method, reliableand stable in performances and easy in storage. According to the invention, the annulus protection fluid for the CO2 flooding injection well and the preparation method thereof are applicable to annular space of oil production wells and gas injection and water injection wells, and can well control the problem of corrosion of annulus oil sleeve pipes.