Soil-cement-bentonite　(SCB)　backfill　has　been　widely　used　in　constructing　cut-off　walls　to　inhibit　groundwater　movement　in　contaminated　sites.　This　study　prepares　SCB　backfill　with　fixed　fluidity.　We　conducted　a　series　of　experiments　to　investigate　the　engineering　characteristics　and　microscopic　mechanism　of　the　backfill.　The　results　indicate　that　the　water　content　in　the　slurry　was　more　sensitive　to　the　bentonite　content.　The　unconfined　compression　strength　(UCS)　value　increased　with　an　increase　in　the　cement　content,　and　the　change　with　an　increase　in　bentonite　content　was　not　noticeable.　The　permeability　coefficient　decreased　distinctly　with　an　increase　in　the　cement　and　bentonite　contents.　The　porosity　of　the　SCB　backfill　increased　with　increasing　bentonite　content　and　decreased　with　increasing　cement　content.　The　UCS　of　SCB　backfill　was　linearly　and　negatively　correlated　with　the　porosity;　the　permeability　coefficient　was　not　significantly　related　to　the　porosity.　The　percentage　of　micro-　and　small-pore　throats　in　the　backfill　increased　with　increasing　bentonite　and　cement　contents.　As　cement　and　bentonite　content　increased　by　6%　in　the　backfill,　the　proportion　of　micro-　and　small-pore　throats　increased　by　0.7%　and　1.2%,　respectively.　The　percentage　of　micro-　and　small-pore　throats　is　deduced　to　be　more　suitable　as　a　characterization　parameter　for　the　permeability　of　the　SCB　backfill.　The　overall　results　of　this　study　show　that　the　reasonably　proportioned　SCB　backfill　has　potential　as　an　eco-friendly　and　cost-effective　material.　Based　on　the　requirements　of　strength　and　permeability　coefficient　(UCS　＞　100　kPa,　28　days　permeability　coefficient　＜1　x　10(-7)　cm/s),　we　suggested　using　a　backfill　with　12%　bentonite　and　9%　cement　as　the　cut-off　wall　mix　ratio.