In this study, the transformation characteristics of sodium in Zhundong coal were investigated, following its combustion (oxidizing atmosphere) and gasification (reducing atmosphere) in a circulating fluidized bed (CFB) experimental system of 0.25 t/d high alkali coal thermochemical conversion. The equilibrium distribution of Na is predicted under the oxidizing and reducing atmospheres by the thermodynamic equilibrium calculation. Na in bottom ash and fly ash evidently decreases as the temperature increases in both the combustion and gasification processes. More Na is retained in the ash during the gasification process. Na exists as sodium aluminosilicate in both combustion bottom ash and gasification bottom ash, and mainly as sodium sulfate (Na2SO4) in combustion fly ash and sodium chloride (NaCl) in gasification fly ash. Na undergoes different transformation processes through combustion and gasification. Gaseous metallic sodium, Na2O and NaCl are released from the coal and condense directly on the cold tube surface during gasification. When SO2 is present, Na reacts with it, forming a large amount of gaseous sodium sulfates during combustion.