In this study, the most abundant C5 carbohydrate unit in biomass, namely xylose, was chosen as the feedstock, and its hydrothermal conversion for furfural production was carried out in a green and renewable solvent system composed of water and butanone, in order to study the role of FeCl₃ and [bmim]Cl of ionic liquid catalyst in the conversion process of xylose. A key intermediate named xylulose from Lewis acid‐catalyzed xylose isomerization was quantified. It was concluded that appropriate content of FeCl₃ and [bmim]Cl favored the isomerization‐dehydration reaction path along which xylose was converted into furfural, while excessive amount of either component would result in side reactions leading to furfural consumption at long reaction times. By comparison between ionic liquid catalysts that had different active metal sites, xylose conversion and furfural yields were found to increase when the Lewis acidity of the metal ions became stronger. Moreover, chlorometallate anions with better catalytic performance than the original neutral salts were formed during catalyst preparation, and in this way, the xylose conversion and furfural formation were further promoted. Finally, the optimized ionic liquid catalyst produced a highest furfural yield of 75% and xylose conversion of 99% at 140°C.