The　thermal　conductivity　of　backfill　materials　directly　affects　the　heat　transfer　efficiency　between　energy　geo-structures　and　the　surrounding　stratum.　Microbially　induced　carbonate　precipitation　(MICP)　possesses　great　potential　for　improving　the　thermal　conductivity　of　backfill　materials.　Magnetic　iron　oxide　nanoparticles　(i.e.,　nano-Fe3O4)　have　been　proven　to　enhance　bacterial　biochemical　activity　by　altering　the　permeability　of　bacterial　biofilms,　thus　potentially　improving　the　MICP　process.　It　was　supposed　to　enhance　the　thermal　conductivity　of　backfill　materials,　allowing　for　applying　energy　geo-structures　in　arid　environments.　In　this　study,　MICP　in　a　solution　environment　was　conducted　to　analyze　bacterial　urease　activity　and　morphology　of　precipitation　at　varying　nano-Fe3O4　contents.　Additionally,　sand　columns　treated　with　MICP　and　different　nano-Fe3O4　contents　were　performed　to　obtain　the　thermal　conductivity　and　unconfined　compressive　strength　(UCS)　through　the　transient　plane　source　(TPS)　method　and　uniaxial　compression　(UC)　experiment.　The　mineral　type,　precipitation　morphology,　and　microstructure　were　identified　using　scanning　electron　microscopy　(SEM)　and　X-ray　diffraction　(XRD).　The　mechanism　of　the　effect　of　nano-Fe3O4　on　bacterial　urease　activity　and　thermal-mechanical　behaviors　was　also　discussed.　The　results　indicated　that　the　nano-Fe3O4　could　enhance　bacterial　urease　activity　and　promote　vaterite　precipitation　in　the　solution　environment.　Conversely,　when　applied　to　MICP-treated　sand,　nano-Fe3O4　could　facilitate　calcite　formation.　Increasing　the　nano-Fe3O4　content　showed　a　positive　correlation　with　increased　thermal　conductivity　and　UCS.　Specifically,　the　optimal　values　of　thermal　conductivity　and　UCS　increased　by　2.42　times　and　2.39　times,　respectively,　compared　to　MICP-treated　specimens　without　nano-Fe3O4.　Microstructure　analysis　revealed　that　calcite　precipitation　at　the　particle　contact　served　a　dual　function:　cementing　particles,　thereby　improving　the　mechanical　strength　and　simultaneously　acting　as　a　“thermal　bridge”　to　enhance　the　thermal　conductivity.　Furthermore,　this　study　provides　a　new　perspective　on　utilizing　magnetized　bacteria　to　reinforce　specific　locations　within　rocks　and　soils　in　the　presence　of　an　external　magnetic　field.　©　2024　Elsevier　Ltd