Application　of　the　medium-deep　borehole　heat　exchanger　(MDBHE)　coupled　ground　source　heat　pump　(GSHP)　system　on　building　heating　has　been　gradually　accepted　by　public　and　its　sustainability　becomes　a　key　concern.　In　this　work,　a　long-term　transient　heat　transfer　model　for　the　MDBHE　coupled　GSHP　system　is　proposed　to　investigate　the　effects　of　various　influencing　factors　on　the　thermal　extraction　and　energy　efficiency　of　this　system.　Sensitive　analysis　highlights　the　significant　effects　of　the　specific　heat　transfer　rate,　rock-soil　thermal　conductivity,　geothermal　gradient,　pipe　depth　and　fluid　velocity　on　the　annual　decline　of　the　system　performance　during　its　long-term　operation.　Furthermore,　the　operation　optimization　for　this　system　should　be　conducted　based　on　its　performance　in　quasi-steady　state.　An　optimal　fluid　velocity　exists　to　acquire　the　maximum　specific　heat　transfer　rate　of　MDBHE　under　a　constant　Seasonal　Performance　Factor　of　Heat　Source　(SPF2).　In　order　to　guarantee　an　efficient　thermal　extraction,　the　comprehensive　effects　of　both　specific　heat　transfer　rate　and　fluid　velocity　on　the　energy　efficiency　should　be　taken　into　account.　For　the　benchmark　MDBHE,　the　operation　condition　is　optimized　with　the　specific　heat　transfer　rate　of　142　W/m,　and　the　fluid　velocity　of　0.7　m/s;　the　annual　SPF2　is　always　greater　than　or　equal　to　4.0　during　the　long-term　operation,　which　ensures　a　sustainable　and　efficient　operation.　©　2020