As the conglomerate reservoir of Lower Karamay Formation in Well530 Area of Xinjiang Oilfield has a complex multi-modal pore structure，rich volcanic tuff，and high iron content，it is difficult to predict the effect of CO₂ flooding field test. With the Lower Karamay Formation reservoir as the research object，the changes in mineral composition，pore structure，fluid ion concentration，and other parameters before and after the reaction of rock fragments，rock powder，core slugs with CO₂ were compared through cast thin section，XRD，XRF，ICP ion concentration analysis，NMR，etc. The controlling factors of the reaction were discussed from the aspects of mineral composition，grain size，physical properties，pore structure，and reaction time. The results show that the impact of CO₂ injection on the reservoir is small. The change in mineral composition shows that the relative content of quartz and clay increases while feldspar and carbonate minerals decrease，and iron minerals precipitate again after dissolution. Elements and pore structure change little，and the average change of porosity is less than 0.5%. The concentration of Ca2+and Mg2+in the liquid phase changes relatively greatly，and the newly added Al3+，Fe3+，Si4+changes slightly. The interaction mechanism between CO₂ and reservoir rocks is the dissolution of feldspar and calcite as well as the first dissolution and then precipitation of iron minerals. As the calcite content of reservoir rock is higher，the variation of pore structure after the reaction is greater. After the reaction，the change in the pore structure of classⅡreservoir is greater than that of classⅢreservoir. The reaction rate of rock powder is stronger than that of rock fragments. The reaction reaches equilibrium within 0-7 days，during which the main action is dissolution. The components of the reaction change little within 7-30 days，and there is the precipitation of Al（OH）3 and Fe（OH）3.