The characteristics of gas-liquid-solid products during corn straw-H2O gasification were studied by the one-stage fluidized-bed/fixed-bed quartz reactor. The results indicate that the transformation between gas and solid products is mainly presented with the increase of feeding time (0-40 min). The aromaticity of biochar decreases slightly in the feeding period of 10-20 min but gradually increases in 20-40 min. The oxygen-containing functional groups in biochar increase gradually with the increase of the feeding time. The K content in biochar increases by 27% in 10-30 min, while that of Ca increases by 36% in 10-20 min. When the carbon conversion rate is less than 8%, the reactivity of biochar produced by 30 min of feeding is highest. The contents of phenol and 2,3-dihydrobenzofuran of tar increase by 25 and 34%, respectively in 20-40 min. The char-tar-H2O interaction in biomass gasification is a two-stage reaction. The first stage is dominated by the volatile- char interactions, while the second stage is mainly due to the char-H2O reaction.