The　Nixu　rock　avalanche　located　in　the　southern　Tibet　of　China　has　been　described　as　one　of　the　typical　large,　rapid　and　long-runout　landslides,　and　its　extraordinary　mobility　have　attracted　great　attention　of　numerous　researchers.　In　order　to　learn　the　movement　mechanism　of　the　Nixu　rock　avalanche,　the　large-scale　high-speed　ring　shear　tests　under　drained　and　undrained　conditions　were　carried　out　on　the　saturated　substrate　materials　taken　from　the　accumulation　zone　of　the　rock　avalanche.　The　results　showed　that　excess　pore-water　pressure　were　generated　during　the　shearing　with　2.5　m/s　velocity　and　the　maximum　excess　pore　pressure　ratio　increased　up　to　0.82　corresponding　to　the　apparent　friction　angle　of　2.15　degrees.　The　observation　and　gain　size　analysis　of　the　shear　zone　in　the　ring　shear　box　showed　that　the　high-speed　shearing　caused　the　pore-water　pressure　and　fine　particle　content　of　the　shear　zone　to　increase　notably,　accompanied　by　a　decrease　of　the　shear　stress　and　effective　friction　coefficient.　The　variations　could　be　attributed　to　the　grain　crushing,　the　migration　of　fine　particles　and　water　to　the　shear　zone　during　the　high-speed　shearing,　resulting　in　the　shear　zone　liquefaction.　This　study　indicates　that　the　＂sliding-surface　liquefaction＂　has　played　a　significant　role　in　promoting　the　rapid　and　long-runout　movement　of　Nixu　avalanche　debris.