Abstract: This paper mainly focused on the effect of steam on char conversion characteristics in typical chemical looping with oxygen uncoupling (CLOU) atmosphere. The effects of particle size, O₂ concentration, reaction temperature and steam concentration on char conversion rate were well studied, the char particle temperature and main species distribution were analyzed in detail. While under low O₂ concentration (1%), the water gasification reaction could increase the char conversion rate; however, at a high O₂ concentration (5%), the contribution of steam to the conversion rate of char was not obvious. Based on the analysis of O₂ distribution and local conversion, it can be concluded that the steam gasification reaction mainly takes place inside the char particle, while oxidation reaction mainly takes place in the external layer of the particle.