During polymer flooding in unconsolidated sandstone reservoirs, the pore structure of the rock varies over time due to fluid seepage effects. Detached particles decelerate and deposit in large pores, leading to a reduction in reservoir permeability. To clarify the time-variation mechanism of the pore structure during polymer flooding in unconsolidated sandstone, four core samples with permeabilities ranging from 1 000×10^-3 μm² to 2 000×10^-3 μm² and similar pore structures were used for displacement experiments. Methods such as X-ray diffraction testing, high-pressure mercury injection testing, and CT scanning testing were employed to investigate changes in mineral composition, porosity, permeability, and pore structure parameters such as pore throat radius before and after polymer flooding. The results indicate that after high-volume and high-speed polymer flooding, clay minerals deposit near the outlet end, increasing by 0.85% compared to the initial content. The maximum pore throat radius decreases by 2.86%, and the average pore throat radius decreases by 3.99%, resulting in a 21.8% reduction in permeability. In terms of pore throat distribution, large pores decrease while medium pores increase. Some large pores become smaller due to particle deposition and blockage, and the pore shape factor approaches 1, indicating reduced pore throat heterogeneity.