In　engineering,　the　new　retaining　walls　are　often　constructed　near　the　existing　structure　owing　to　the　space　limitation.　The　backfill　behind　the　retaining　wall　is　narrow,　which　causes　an　overestimation　in　the　active　earth　pressure　by　using　Coulomb＇s　earth　pressure　theory.　In　previous　studies,　experimental　observations　for　the　failure　modes　of　narrow　backfills　are　still　rare.　To　confirm　the　failure　mode　of　the　narrow　backfill,　the　experimental　method　and　the　geotechnical　particle　image　velocimetry　method　are　employed　to　observe　the　active　failure　process　of　the　cohesionless　narrow　backfill　with　various　widths　under　the　translation　mode.　The　experimental　results　revealed　that　the　decrease　in　the　length　of　the　backfill　width　led　to　the　increase　in　the　inclined　angle　of　the　sliding　surface.　When　the　backfill　width　was　sufficiently　small,　the　sliding　surface　developed　from　the　wall　toe　to　another　wall　face,　and　then　another　sliding　surface　occurred　as　a　reflection.　In　addition,　the　active　earth　pressure　of　the　narrow　backfill　is　significantly　smaller　than　that　calculated　using　Coulomb＇s　method.　The　active　failure　calculation　models　are　established　based　on　the　experimental　results.　The　active　earth　pressure　of　the　narrow　cohesionless　backfill　under　the　translation　mode　is　derived　by　using　the　limit　equilibrium　methods.　The　proposed　method　was　validated　by　comparing　with　the　previous　method　and　the　experimental　data.