<p>The pore structure and strength of cement mortar can be significantly affected by prolonged exposure to moisture, posing a threat to the long-term stability of construction projects. To investigate this, tests were conducted using five different grain sizes of crushed stone sand (CRS) aggregate. The coupling of these grain sizes to form a synthetic gradation was then compared. Based on the nuclear magnetic resonance (NMR) technique, the pore structure and mechanical strength characteristics of the mortar after water erosion were explored, and the correlation between them was obtained. The influence law of the pore structure and mechanical strength of the aggregate grade paired mortar was analyzed, and the variation amount Δ was introduced to characterize the erosion ability of water on the pore structure and mechanical strength of the mortar. The results showed that the total porosity of mortar increased after water erosion, and the more developed the pore structure, the relatively stronger the erosion effect. Among them, the dissolution of Ca(OH)₂ and CH, as well as the filling of the pore space by hydration products, significantly increased the micropores and clay-bonded fluid pores. When the total porosity increased, the connectivity between pores gradually increased, and the complexity of pores gradually decreased. The fitting results of the two-factor (surface-to-volume ratio (SVR) and aggregate packing density (<i>ρ</i>_d)) of aggregate grading with the macro- and micro-scale characteristics of mortar were higher than those of one-factor (SVR or <i>ρ</i>_d), and the formation of the macro- and micro-scale characteristics was influenced by both SVR and <i>ρ</i>_d.</p>