Surface　functionalization　has　been　considered　as　an　effective　strategy　to　manipulate　charge　separation　of　carbon　nitride　and　therefore　to　largely　improve　the　photocatalytic　H　2　evolution　efficiency.　Poly　heptazine　imide　(PHI)　is　a　new　class　of　crystalline　carbon　nitride　frameworks　that　exhibits　remarkable　photocatalytic　performance　for　hydrogen　evolution.　To　further　improve　the　H　2　evolution　performance　of　PHI　and　explore　the　reaction　mechanism,　ammonium　thiocyanate　was　used　as　a　precursor　for　the　synthesis　of　poly　heptazine　imide　at　elevated　temperatures　under　molten　salt　conditions.　The　optimized　PHI　with　an　abundance　of　surface　cyano　groups　shows　a　significantly　enhanced　photocatalytic　performance　for　H　2　evolution,　which　is　4.3　times　that　on　pristine　PCN.　Most　importantly,　the　surface　cyano　group　adjusts　the　electron　intensity　of　the　polymeric　framework,　enhances　the　light　absorbance,　reduces　the　bandgap,　and　improves　the　charge　separation　efficiency.　The　synthetic　technique　also　could　be　applied　to　other　sulfur　-containing　precursors　for　the　synthesis　of　PHI　frameworks　with　excellent　hydrogen　evolution　production　performance.