Under　the　background　of　climate　warming　in　the　Qinghai-Tibetan　Plateau　(QTP),　frequent　freeze–thaw　cycling　(FTC)　brings　about　great　geological　disasters　such　as　subgrade　failure,　landslides,　and　mudslides,　which　is　closely　related　to　the　strength　reduction　caused　by　the　structural　damage　of　soils.　In　this　study,　to　explore　the　association　between　macro　shear　strength　and　microstructure　evolution　of　soils　subjected　to　FTC,　the　red　clay　distributed　widely　in　the　QTP　was　chosen　and　used　to　conduct　a　series　of　triaxial　shear　and　nuclear　magnetic　resonance　(NMR)　tests　in　the　range　of　1　to　7　FTCs.　Triaxial　shear　test　results　reveal　that　the　shear　strength　reduction　of　specimens　mainly　occurs　within　five　FTCs,　and　the　trend　of　peak　deviator　stress　with　increasing　FTCs　can　be　described　in　three　stages:　rapid　descent　(FTCs　less　than　three),　slow　descent　(FTCs　between　three　and　five),　and　stabilization　(FTCs　greater　than　five).　NMR　tests　show　that　the　T2　spectrum　curves　exhibit　a　distinct　bimodal　distribution　characteristic,　corresponding　to　macropores　and　micropores.　Part　of　the　micropores　gradually　develop　into　macropores　with　increasing　FTCs,　especially　within　five　FTCs.　The　increase　in　macropores　proportion　leads　to　a　loose　soil　structure,　which　is　consistent　with　the　deterioration　of　the　shear　strength　of　specimens.　Finally,　based　on　the　experimental　results　and　classical　Mohr–Coulomb　theory,　a　new　shear　strength　model　with　structural　damage　for　red　clay　has　been　proposed　by　introducing　a　damage　factor　expressed　by　T2　spectral　area.　©　2024　by　the　authors.