The　technology　of　supercritical　water　gasification　(SCWG)　of　coal　has　a　great　prospect　because　it　converts　coal　into　hydrogen-rich　gas　products　efficiently　and　cleanly.　However,　there　are　bottlenecks　affecting　the　complete　gasification　of　coal　in　supercritical　water　(SCW)　without　catalyst　under　moderate　conditions.　This　work　is　to　explore　the　restricted　factor　for　complete　gasification　of　coal　in　SCW　by　investigating　the　conversion　mechanism.　The　conversion　mechanism　of　SCWG　of　coal　with　and　without　K2CO3　is　proposed.　Polycyclic　aromatic　hydrocarbons　(PAHs)　with　graphite　phase　structures　are　formed　by　the　condensation　of　aromatic　structures　at　550–750　°C.　It　is　the　restricted　factor　due　to　its　characteristic　of　difficulty　to　be　gasified.　There　is　no　condensation　of　aromatic　structures　in　the　process　of　SCWG　of　coal　with　K2CO3,　which　effectively　inhibited　the　formation　of　PAHs　with　graphite　phase　structures.　K2CO3　dramatically　promoted　the　SCWG　of　coal,　leading　to　carbon　gasification　efficiency　(CE)　reaching　98.43%.　©　2021　Hydrogen　Energy　Publications　LLC