The effect of O-2/C ratio on gasification performance and sodium transformation behavior of a high-sodium Zhundong coal was studied in a novel fluidized bed reactor. The sodium distribution was predicted through thermodynamic equilibrium calculation, and the possible reactions between sodium and other compounds were also discussed. The experimental results show that no agglomeration occurred in all cases. The carbon conversion and gas yield increase with the rise of O-2/C ratio, while the cold gas efficiency has a maximum point (66.0%) at the O-2/C molar ratio of 0.34. With the rise of the O-2/C ratio, H2O-soluble sodium content and HCl-insoluble sodium content in fly ash and bottom char decrease, whereas HCl-soluble sodium content and CH3COONH4-soluble sodium content increase. Sodium presents mainly as NaAlSiO4, NaAlSi2O6 and Na2SO4 in bottom char, and as NaCl and NaAlSiO4 in fly ash. The equilibrium calculation results are coherent with the experimental study and can be used to predict the sodium transformation for the utilization of high sodium coal. The Si/Na ratio could affect the transformation of sodium significantly, and the agglomeration phenomenon could be reduced or avoided by inducing the formation of high melting point sodium-containing compounds through controlling the ratio of Si/Na.