During the process of CO₂ being injected into deep saline aquifers, residual water formed in rock pore has negative impacts on CO₂ injectability, storage capacity and safety. Therefore, it is important to investigate the formation and evolution of residual water under various influencing factors. In this study, natural rock core taken from the deep reservoirs of the Ordos Basin were used to conduct core-flooding experiments using 3 sets of CO₂ purity (99.999%CO₂, 75%CO₂+25%N2 and 50%CO₂+25%N2) at 40℃ and 8 MPa to investigate the effect of CO₂ purity on residual water. The experimental results show that the order of time required to reach the breakthrough point and the drainage endpoint are, 99.999%CO₂<75%CO₂+25%N2<50%CO₂+50%N2; the order of the irreducible water saturations is: 99999%CO₂<75%CO₂+25%N2<50%CO₂+25%N2. The analysis revealed that changes in CO₂ purity lead to changes of important properties in two-phase floods, such as interfacial tension, wettability, and viscosity ratio. By analyzing the LogCa-LogM displacement stability diagram, the capillary force was determined to be the dominant factor affecting the results of experiments. This study is of great value for predicting residual water saturation under various conditions and evaluating CO₂ sequestration capacity.