Graphitic　carbon　nitride　(CxNy)　has　a　two-dimensional　structure　similar　to　N-doped　graphene,　which　is　nitrogen　-　rich　and　contains　porous　defect　sites　that　can　serve　as　atomic　storage　sites　for　alkali　metals.　Compared　with　C3N4,　the　most　widely　reported　carbon　nitride　family,　g-CN　is　structurally　stable　without　graphitic　nitrogen.　However,　the　adsorption　energy　of　Li/Na/K　in　the　pore　size　is　too　negative　(-2.60/-2.99/-3.11　eV)　to　be　desorbed,　and　the　high　diffusion　barrier　of　Li/Na/K　ion　(2.94/3.11/2.43　eV)　implies　the　low　charge/discharge　rate　for　metal-ion　batteries.　Here,　we　showed　that　doping　B　atoms　into　the　pore　sites　of　g-CN　(BC3N3)　possess　some　advantages　for　application　in　batteries.　Specifically,　the　BC3N3　monolayer　has　excellent　stability,　with　moderate　adsorption　ability　for　Na/K　(-0.54/-0.98　eV)　and　a　much　lower　diffusion　barrier　of　ion　(0.73/0.35　eV).　BC3N3　shows　excellent　theoretical　specific　capacity　(603.30/904.95　mAh/g)　and　low　average　open-circuit　voltage　as　an　anode　material　for　Na/K-ion　batteries.　Moreover,　under　a　small　amount　of　alkali　metal　adsorption,　BC3N3　exhibits　metallic　properties　and　the　electrical　conductivity　of　BC3N3　is　improved.　These　favorable　properties　indicate　that　the　B-doped　g-CN　monolayer　is　a　promising　anode　material　for　Na/K　ion　batteries　and　this　approach　can　be　extended　to　other　porous　carbon-nitrogen　structures.