Manganese-based　chalcogenides　have　significant　potential　as　anodes　for　sodium-ion　batteries　(SIBs)　due　to　their　high　theoretical　specific　capacity,　abundant　natural　reserves,　and　environmental　friendliness.　However,　their　application　is　hindered　by　poor　cycling　stability,　resulting　from　severe　volume　changes　during　cycling　and　slow　reaction　kinetics　due　to　their　complex　crystal　structure.　Here,　an　efficient　and　straightforward　strategy　was　employed　to　in-situ　encapsulate　single-phase　porous　nanocubic　MnS0.5Se0.5　into　carbon　nanofibers　using　electrospinning　and　the　hard　template　method,　thus　forming　a　necklace-like　porous　MnS0.5Se0.5-carbon　nanofiber　composite　(MnS0.5Se0.5@N-CNF).　The　introduction　of　Se　significantly　impacts　both　the　composition　and　microstructure　of　MnS0.5Se0.5,　including　lattice　distortion　that　generates　additional　defects,　optimization　of　chemical　bonds,　and　a　nano-spatially　confined　design.　In　situ/ex-situ　characterization　and　density　functional　theory　calculations　verified　that　this　MnS0.5Se0.5@N-CNF　alleviates　the　volume　expansion　and　facilitates　the　transfer　of　Na+/electron.　As　expected,　MnS0.5Se0.5@N-CNF　anode　demonstrates　excellent　sodium　storage　performance,　characterized　by　high　initial　Coulombic　efficiency　(90.8%),　high-rate　capability　(370.5　mAh　g(-1)　at　10　A　g(-1))　and　long　durability　(over　5000　cycles　at　5　A　g(-1)).　The　MnS0.5Se0.5@N-CNF　//NVP@C　full　cell,　assembled　with　MnS0.5Se0.5@N-CNF　as　anode　and　Na3V2(PO4)(3)@C　as　cathode,　exhibits　a　high　energy　density　of　254　Wh　kg(-1)　can　be　provided.　This　work　presents　a　novel　strategy　to　optimize　the　design　of　anode　materials　through　structural　engineering　and　Se　substitution,　while　also　elucidating　the　underlying　reaction　mechanisms.