Cellulose acetate butyrate (CAB) with butyryl content of 6-47 wt % was homogeneously synthesized in 1-allyl-3-methylimidazolium chloride (AmimCl) in a single step without using any catalysts. The effects of different acylating agents (acetic anhydride and butyric anhydride) addition methods and reaction conditions on the acyl content of CABs were investigated. Synthesized CABs were characterized by FTIR, NMR, solubility, and thermal analysis. The acylating agents addition method influences the butyryl content, substituent distribution within the anhydroglucose units (AGU), and the properties of CAB. The CAB obtained with butyryl content greater than 38% is completely soluble in 2-methyl-ethyl ketone, 1,2-dichloromethane, ethyl acetate, and butyl acetate. After the mix-acylation, AmimCl can be easily recycled and reused. This study provides a novel way for the dean production of CAB under mild conditions for future industrial applications.