The　development　of　efficient,　low-cost,　and　eco-friendly　catalysts　for　nitrogen　fixation　is　essential　and　provides　an　alternative　method　to　the　traditional　Haber-Bosch　process.　However,　studies　on　thermal　catalyst　of　nitrogen　fixation　mainly　focus　on　metal-containing,　and　the　microscopic　mechanism　of　thermal　reduction　process　is　still　limited.　Herein,　we　explored　an　economic　metal-free　boron　atom　decorated　poly(triazine　imide)　(B/PTI),　a　crystalline　carbon　nitride,　as　an　excellent　thermal　catalyst　of　nitrogen　fixation　and　proposed　a　substrate-hydrogen　mechanism　for　the　N-2　thermal　reduction　reaction　(NTRR).　Our　results　reveal　that　the　substrate　hydrogen　as　the　hydrogen　source　can　promote　the　hydrogenation　process　with　activation　barrier　of　0.56　eV,　significantly　lower　than　that　of　reported　NTRR　catalysts.　Importantly,　the　B/PTI　exhibits　high　turnover　frequency,　which　is　comparable　to　Fe,　Ru,　and　Ti　catalysts.　Our　work　offers　new　insights　into　NTRR　mechanism　and　provides　an　alternative　solution　for　the　sustainable　ammonia　synthesis.