Pre-extracting part of hemicelluloses in wood can not only realize a high value-added utilization of hemicelluloses but also ensure the production of traditional pulping using the extracted wood raw materials, which benefits the highly efficient utilization of bioresources. In this work, this concept was applied to the poplar chemi-thermomechanical pulping. The chemical components of the autohydrolysis liquor (AHL) at different autohydrolysis pretreatment conditions were studied, and the formation of acetic acid and its effect on the dissolution of carbohydrate were also evaluated. The results show that the content of the sugars (xylose, glucose, and arabinose), acetic acid, and furfural increased with the increase of autohydrolysis pretreatment temperature and time, and the dissolved sugars in the Al-IL were mostly in a polymeric form. The release of acetic acid played an important role in the dissolution of carbohydrate and the pH value of the AHL.