Heteroatom-doping　is　a　promising　strategy　to　tuning　the　microstructure　of　carbon　material　toward　improved　electrochemical　storage　performance.　However,　it　is　a　big　challenge　to　control　the　doping　sites　for　heteroatom-doping　and　the　rational　design　of　doping　is　urgently　needed.　Herein,　S　doping　sites　and　the　influence　of　interlayer　spacing　for　two　kinds　of　hard　carbon,　perfect　structure　and　vacancy　defect　structure,　are　explored　by　the　first-principles　method.　S　prefers　doping　in　the　interlayer　for　the　former　with　interlayer　distance　of　3.997　Å,　while　S　is　doped　on　the　carbon　layer　for　the　latter　with　interlayer　distance　of　3.695　Å.　More　importantly,　one　step　molten　salts　method　is　developed　as　a　universal　synthetic　strategy　to　fabricate　hard　carbon　with　tunable　microstructure.　It　is　demonstrated　by　the　experimental　results　that　S-doping　hard　carbon　with　fewer　pores　exhibits　a　larger　interlayer　spacing　than　that　of　porous　carbon,　agreeing　well　with　the　theoretical　prediction.　Furthermore,　the　S-doping　carbon　with　larger　interlayer　distance　and　fewer　pores　exhibits　remarkably　large　reversible　capacity,　excellent　rate　performance,　and　long-term　cycling　stability　for　Na-ion　storage.　A　stable　and　reversible　capacity　of　≈200　mAh　g　−1　is　steadily　kept　even　after　4000　cycles　at　1　A　g　−1　.　©　2018　WILEY-VCH　Verlag　GmbH　&　Co.　KGaA,　Weinheim