Na2TiSiO5　(NTSO)　is　a　low-cost　Li-ion　battery　anode　with　great　application　potential,　such　as　the　tetragonal　NTSO　(T-NTSO)　with　a　high　capacity　and　a　low　voltage.　In　addition　to　the　tetragonal　structure,　NTSO　has　two　other　polymorphs.　However,　the　basic　understanding　of　the　structure,　ion　insertion　and　transport　mechanisms　of　these　new　materials　is　still　lacking.　Herein,　we　present　a　combined　experimental　and　computational　investigation　of　the　tetragonal　and　orthorhombic　NTSO　to　reveal　the　intrinsic　mechanism　leading　to　the　superior　electrochemical　performance　of　T-NTSO.　We　determined　that　the　insertion　site　with　a　flexible　Ti4+/Ti3+　redox　pair　is　critical　for　Li+　insertion　stability.　The　large　number　of　such　flexible　sites　in　the　T-NTSO　results　in　a　higher　capacity　and　higher　ionic　conductivity　than　those　of　orthorhombic　polymorph.　The　understanding　of　intrinsic　properties　will　accelerate　the　development　and　utilization　of　titanosilicates　as　the　next　generation　low-voltage　anode　of　Li-ion　battery.