CO₂主导宏观驱油，非混相条件下纳米聚硅强化微观洗油，可共同实现采收率提升。 In view of the low recovery of tight low-permeability oil reservoirs by water flooding in a certain block of Changqing Oilfield，a supercritical CO₂-nano-polysilicon composite displacement fluid was developed to explore its oil displacement mechanism and the effect of enhanced oil recovery. Poly（2-acrylamido-2-methylpropane sulfonic acid）（PAMPS）was used for insitu surface modification of SiO₂ to prepare nano-polysilicon particles with strong adsorption capacity and interfacial activity.Through core displacement experiments，the oil displacement performance of composite systems of nano-polysilicon with different mass fractions（1%-5%）and supercritical CO₂ was compared under miscible and immiscible conditions. The recovery change was monitored in the experiments，and the effects of nano-polysilicon on rock wettability，interfacial tension，and flow characteristics were analyzed. Under miscible conditions，supercritical CO₂ dominated the oil displacement process. The recovery of water flooding was only 32.7%，while the composite system increased it to over 70.0%. Among them，low-dose nano-polysilicon （1%-3%）significantly expanded the swept area of the oil phase and improved micro-scale oil displacement efficiency by enhancing the viscoelasticity and interfacial activity of the displacement fluid. However，high-dose nano-polysilicon（4%-5%）was prone to blocking flow channels due to particle accumulation，leading to a sharp increase in displacement pressure and a slowdown in recovery growth. Under immiscible conditions，nano-polysilicon played a more critical role. It reduced the interfacial tension between supercritical CO₂ and crude oil，increased fluid viscosity，and synergistically prolonged the shut-in time，promoting more nano-polysilicon to adsorb on the oil-bearing rock surface，transforming the oil-wet surface into a water-wet or neutral-wet state，and stripping residual oil phases. Under these conditions，the recovery increased from 31.6% of water flooding to 50%-60%. The surface modification of nano-polysilicon gave it amphiphilicity，enabling it to form a stable adsorption layer at the rock，oil，and water interface and weaken the adhesion between crude oil and rock. Supercritical CO₂ formed miscible displacement by mixing with crude oil. The synergistic effect of the two significantly improved oil washing efficiency. Supercritical CO₂ and nano-polysilicon complemented each other in displacement. CO₂ dominated macro-scale oil displacement under miscible conditions，while nano-polysilicon strengthens micro-scale oil washing under immiscible conditions，jointly achieving recovery improvement.