Gel particle profile modification and flooding is one of the main technical methods for stabilizing oil, water control, and production cycle prolonging in the oil field. Quantitative evaluation of the plugging effect of the displacement agent on pores of different sizes under various conditions may provide useful guidance for field development. In this study, nuclear magnetic resonance technology and dynamic physical simulation experiments were used to quantitatively evaluate the plugging effect of pore spaces at different scales. The influence of permeability, temperature, injection pressure, injection volume, and formation water salinity on plugging effect was also investigated. The results show that the poly(ethylene glycol) (PEG)-1 gel particle system flooding agent can effectively plug pore spaces at different scales. For artificial cores, the plugging dimension was between 0.10 and 59.95 ms, and for natural cores, the plugging dimension was between 0.10 and 1144.64 ms. The plugging effect of gel particles was negatively correlated with permeability and salinity of formation water and positively correlated with temperature, injection pressure, and injection volume. Moreover, the plugging efficiency of gel particles for larger pores (>10.0 ms) was always better than that for smaller pores (0.10-10.0 ms).