<p>CO₂ flooding, as an effective method for developing low-permeability heavy oil reservoirs, is often limited by the early gas breakthrough due to the high mobility of the gas. To address this issue, research on enhanced oil recovery using a composite flooding method combining viscosity reducer and CO₂ was conducted. Firstly, a novel low molecular weight viscosity reducer, PAINS, was prepared. Subsequently, the mechanism of viscosity reduction by the viscosity reducer in heavy oil was explored from a molecular simulation perspective. Finally, the oil displacement efficiency of the viscosity reducer-CO₂ combination flooding was studied through core flooding experiments. The results showed that the prepared viscosity reducer exhibited effective viscosity reduction; within the viscosity reducer-heavy oil system, the viscosity reducer achieved its effect by altering the internal structure of the heavy oil. The slug of the viscosity reducer was able to improve the mobility ratio of oil and gas caused by viscosity differences, delaying the breakthrough of CO₂ along gas flow channels. The viscosity reducer-CO₂ combination flooding method increased oil recovery by 14.5% compared to single CO₂ flooding, indicating that the viscosity reducer-CO₂ combination flooding technology has broad application prospects in the development of low-permeability heavy oil reservoirs.</p> Graphical abstract <p></p>