Inverted　T-type　retaining　walls　are　commonly　used　in　subgrade　or　slope　support　engineering,　which　inevitably　satisfies　a　narrow　backfill.　Using　the　classical　earth　pressure　calculation　method　in　a　narrow-backfill　case　causes　an　inevitable　error.　The　current　narrow-backfill　earth　pressure　theory　does　not　apply　to　inverted　T-type　retaining　walls.　In　this　study,　the　failure　mechanism　in　a　narrow　backfill　when　the　inverted　T-type　retaining　wall　rotates　about　the　heel　is　investigated　using　adaptive　finite　element　analysis　method.　Numerical　analysis　reveals　multiple　sliding　surfaces.　A　theoretical　model　for　calculating　earth　pressure　using　difference　and　limit　equilibrium　methods　is　proposed.　The　proposed　model　is　suitable　for　more　complex　conditions,　including　narrow　backfill,　irregular　ground,　and　non-uniform　overload,　than　previous　models.　Parameter　analysis　reveals　that　the　cross-sectional　area　of　the　plastic　zone　and　active　earth　pressure　have　a　positive　correlation.　Further,　the　interface　friction　influences　the　decrease　in　active　earth　pressure.　Fitting　formulas　for　assessing　the　cases　of　long　and　short　heel　and　the　critical　size　of　backfill　width　are　presented　to　facilitate　practitioners　to　evaluate　the　backfill.