Poplar wood fibers (WFs) were pretreated with ionic liquid (IL) 1-butyl-3-methylimidazolium chloride ([C4mim]Cl) at 125 °C for 1 h, and the influence of the wood loadings (5%, 15%, 30%, and 50%) on the pretreatment efficiency and downstream acetylation was studied. The crystallinity and lignin content of the WFs decreased after IL pretreatment at low wood loadings, especially at 5%. Wood fiber acetylation was performed under a molar ratio (acetic anhydride/–OH in the WFs) of 2:1, with toluene as the reaction medium and pyridine as the acid capturer. Ionic liquid pretreatment at 5% and 15% loading greatly contributed to the increase of weight percent gain (WPG) after acetylation, leading to much higher reaction efficiency, or lower energy consumption. The acetylated products that underwent pretreatment (mainly at 5% loading) had slightly higher thermal stability than those that did not undergo pretreatment. The crystallinity and moisture sorption ability of the products were determined primarily by the WPG value.