The discrete-element method (DEM) was employed to simulate undrained cyclic triaxial tests on clean sands with different particle gradations, mixed with varying amounts of non-plastic fines. The results highlight that both fines content (FC) and the uniformity coefficient of host sand (Cus) significantly influence the liquefaction resistance and force transmission within contact network. Microscopic analysis reveals that sand-sand contacts dominate deviatoric stress contributions, whereas fines-fines contacts, despite their abundance, have minimal influence. To better characterize the role of fines in stress transmission, a new contact state variable, the soil skeleton coordination number (MCNsk), is proposed. This parameter effectively identifies active contacts within the soil skeleton and provides a unified framework for assessing the cyclic liquefaction resistance of granular mixtures, regardless of particle size distribution. The integration of macro- and micro-scale analyses enhances the understanding of the role of fines and its effect on the liquefaction resistance of sand-fines mixtures.