Abstract: In order to better apply starch as a polymer material in the material industry, it is necessary to modify starch. In this study, cassava starch was used as raw material, and it was pretreated and acetylated in an anhydrous medium. By controlling the change in reaction time, acetylated starch with different degrees of substitution was prepared. And the effects of pretreatment on the physicochemical properties of acetylated starch particles, as well as the differences in physicochemical properties between high degree of substitution starch and low degree of substitution starch, were investigated through characterization methods such as X-ray diffraction, polarizing microscope, scanning electron microscope, contact angle measurement, and differential scanning calorimetry analysis. The results showed that pre-treatment could shorten the time required for acetylation of raw starch. The crystal diffraction peak of high degree of substitution acetylated starch disappeared, the polarized cross disappeared, and the morphology changed from circular to porous. It could be dissolved in organic solvents such as acetone. After 0.5 hours of acetylation reaction, the hydrophobic contact angle of pretreated starch could reach (69.006±0.520)°, the starch melting temperature decreased to 155 ℃, and the decomposition temperature increased from 280 ℃ to 340 ℃, enhanced the thermal stability of starch and possessed thermoplastic properties. This study can prepare acetylated starch with a degree of substitution of 2.5 within half an hour, which can be dissolved in organic solvents, providing new ideas for accelerating the application of starch in the material industry.