Understanding the storage mechanisms in CO₂ flooding is crucial, as many carbon capture, utilization, and storage (CCUS) projects are related to enhanced oil recovery (EOR). CO₂ storage in reservoirs across large timescales undergoes the two storage stages of oil displacement and well shut-in, which cover multiple replacement processes of injection–production synchronization, injection only with no production, and injection–production stoppage. Because the controlling mechanism of CO₂ storage in different stages is unknown, the evolution of CO₂ storage mechanisms over large timescales is not understood. A mathematical model for the evaluation of CO₂ storage, including stratigraphic, residual, solubility, and mineral trapping in low-permeability tight sandstone reservoirs, was established using experimental and theoretical analyses. Based on a detailed geological model of the Huaziping Oilfield, calibrated with reservoir permeability and fracture characteristic parameters obtained from well test results, a dynamic simulation of CO₂ storage for the entire reservoir life cycle under two scenarios of continuous injection and water–gas alternation were considered. The results show that CO₂ storage exhibits the significant stage characteristics of complete storage, dynamic storage, and stable storage. The CO₂ storage capacity and storage rate under the continuous gas injection scenario (scenario 1) were 6.34 × 10⁴ t and 61%, while those under the water–gas alternation scenario (scenario 2) were 4.62 × 10⁴ t and 46%. The proportions of storage capacity under scenarios 1 and 2 for structural or stratigraphic, residual, solubility, and mineral trapping were 33.36%, 33.96%, 32.43%, and 0.25%; and 15.09%, 38.65%, 45.77%, and 0.49%, respectively. The evolution of the CO₂ storage mechanism showed an overall trend: stratigraphic and residual trapping first increased and then decreased, whereas solubility trapping gradually decreased, and mineral trapping continuously increased. Based on these results, an evolution diagram of the CO₂ storage mechanism of low-permeability tight sandstone reservoirs across large timescales was established.