Broadband　near　-infrared　(NIR)　phosphors　play　a　crucial　role　in　the　development　of　next　-generation　smart　NIR　light　sources　based　on　blue　light　-emitting　diodes　(LEDs).　The　emission　bandwidth　is　an　important　parameter　for　evaluating　the　performance　of　these　phosphors.　Herein,　an　ultra　-broadband　NIR　phosphor　CaMgSi2O6:Cr3+　(CMSO:Cr),　with　a　tunable　emission　band　and　a　full　width　at　half　maximum　(FWHM)　ranging　from　138　nm　to　393　nm,　is　obtained　by　adjusting　the　doping　concentration　of　Cr3+.　In　the　low　-concentration　doped　sample,　a　single　emission　peak　at　785　nm　is　observed.　When　the　concentration　of　Cr3+　exceeds　1.0　at%,　emissions　at　both　785　nm　and　980　nm　are　detected.　The　emission　peak　at　785　nm　originates　from　the　[MgO6](I)　octahedron　within　the　CMSO　structure.　On　the　other　hand,　the　emission　peak　at　980　nm　is　attributed　to　the　presence　of　a　twisted　octahedron　[MgO6](II),　which　is　caused　by　defects　and　vacancies　resulting　from　the　charge　imbalance　when　Mg2+　ions　are　replaced　by　Cr3+　ions.　The　Cr3+　ion　primarily　occupies　the　[MgO6](I)　site　in　the　lowerconcentration　doped　sample,　which　explains　the　photoluminescence　peak　at　785　nm.　With　the　increase　in　the　concentration　of　Cr3+　ions,　more　and　more　Cr3+　ions　preferentially　occupy　the　[MgO6](II)　octahedral　sites,　leading　to　the　broad　emission　at　980　nm.　A　novel　NIR　phosphor　light　source　was　fabricated　by　combining　a　blue　chip　with　CMSO:Cr　phosphor.　Driven　by　a　current　of　60　mA,　the　NIR　light　output　power　of　5.89　mW　and　0.76　mW　is　achieved　by　using　CMSO:0.015Cr　and　CMSO:0.04Cr,　respectively.　This　work　demonstrates　that　new　luminescent　centers　can　be　generated　by　substituting　heterovalent　ions,　resulting　in　broadened　emission　spectra.