Abstract: The necessity and feasibility of EOR in old fields were analyzed and the basic concept and approaches for old fields’secondary development were elaborated, including the main mechanism, mode and efficiency of CO2 flooding and the related researches and experimental progress at home and abroad, as well as the prospects of such secondary development and technologies for CO2 flooding. The core of secondary development or "redevelopment" is to rebuild new development systems by means of new development ideas and new technologies in order to substantially tap potentials and improve ultimate oil recovery of the old or maturing oilfields, including reconstructing the geological knowledge, the well pattern and ground technological process, etc. The practices show that CO2 flooding can not only improve the old oilfields’recovery, but also restore carbon and protect environment. The researches of related theories and technologies have been carried out effectively and shown good results at home and abroad. The combination of old field redevelopment and CO2 flooding is the trend of EOR and one of the key technologies in China in the future