Abstract: Low field ¹H-nuclear magnetic resonance (NMR) is a quick, effective and nondestructive measuring technique to investigate the distribution and content of water by H-atom energy changes. Transverse relaxation time (T₂) and seepage amount of redispersible polymer powder (polymer)-modified repair mortars were obtained by triaxial low field ¹H-NMR, that was wont to analyze the permeability of the repair mortars. The mechanism of polymer influence on the transverse relaxation characteristics of repair mortars was explained by pore structure and micromorphology analysis. It’s shown that: Osmotic pressure is the main factor affecting the transverse relaxation time and seepage amount of mortar. Polymer reduces the water content in capillary and gel pores of repair mortar under different osmotic pressures, and the water seepage decreases by 57.3% at 6MPa, which significantly improves the impermeability of repair mortar. The polymer improves the toughness of the repair mortar, which makes the cross-cracking damage of mortar change into compression deterioration under three-axis loading conditions, thus avoiding the breakthrough of impermeability of the mortar under axial compression. The blocking and filling effect of polymer film reduces the porosity of mortar by 3.28%, and weakens the transmission ability in gel pores and capillary pores. Therefore, the transverse relaxation strength and water seepage of repair mortar are reduced, enhancing its impermeability. Compared with the traditional impermeability testing methods, low field ¹H-NMR can characterize the water infiltration process and the seepage amount at different osmotic pressures, which is a practical impermeability testing method.