Equiatomic　high-entropy　alloys　(HEAs)　fabricated　by　additive　manufacturing　(AM)　have　gained　increasing　attention　since　they　present　superior　mechanical　properties　over　their　counterparts　made　by　traditional　synthesis　routes.　However,　few　studies　have　investigated　AM　of　non-equiatomic　HEAs,　although　this　type　of　HEAs　showing　excellent　mechanical　properties.　In　this　paper,　we　additively　manufactured　a　novel　non-equiatomic　FeCoCrNi　HEA　via　in-situ　alloying　316L　stainless　steel　and　CrCoNi　medium-entropy　alloy.　Besides　the　fine　grain　size　(9　μm)　and　high　dislocation　density　(3.8　×　1014　m−2),　we　observed　unexpected　high-intensity　of　hybrid　crystal-amorphous　precipitates,　which　have　never　been　reported　in　NiCoCr-based　alloys　made　by　AM.　These　unique　microstructures　led　to　exceptional　combination　of　strength　(561　MPa)　and　elongation　(41　%)　exceeding　those　of　additively　manufactured　equiatomic　FeCoCrNi　HEAs.　Our　observation　from　EBSD,　TEM　and　atomistic　simulations　reveals　a　group　of　deformation　mechanisms　of　stacking　faults,　deformation　twins,　TB-dislocation　and　dislocation-precipitates　interactions.　Our　findings　not　only　provide　new　insights　into　AM　of　non-equiatomic　HEAs　via　in　situ　alloying,　also　shed　lights　on　understanding　the　microstructure-properties　relationship　of　non-equiatomic　HEAs　fabricated　by　AM.　©　2024　The　Author(s)