The　mechanism　behind　the　growth　of　carbon　nanotubes　(CNTs)　based　on　the　microwave　plasma　chemical　vapour　deposition　(MPCVD)　method　is　clarified.　Herein,　the　influence　of　nitrogen　(N-2)　and　hydrogen　(H-2)　concentration/flux　on　the　CNT　growing　process　has　been　systematically　studied.　First-principles　calculations　indicate　that　the　reaction　energy　of　the　dissociative　carbon　obtained　from　CH4　is　significantly　decreased　when　plasma　is　utilised.　Values　of　2.87　(or　lower)　and　0.18　eV　with　the　introduction　of　N　plasma　and　H　plasma,　respectively,　are　lower　than　the　18.31　eV　obtained　for　CH4　splitting　without　plasma　assistance.　This　indicates　that　N　plasma　and　H　plasma　could　promote　the　growth　of　CNTs.　Hence,　a　novel　coupling　enhanced　mechanism　for　improved　understanding　of　CNT　growth　is　proposed.　For　the　catalyst　activity,　N　plasma　helps　to　rapidly　activate　the　catalyst,　and　H　plasma　maintains　the　activated　state　of　the　catalyst　for　longer.　This　model　matches　well　with　experimental　results.　Based　on　this　mechanism,　different　morphologies　of　MWCNTs　can　be　modulated,　which　could　serve　as　a　good　supporter　for　sensor　materials　at　room　temperature.