Esterified starch was prepared by nonenyl succinic anhydride as esterification agent, 4-dimethylaminopyridine (DMAP) as catalyst, and ionic liquid (1-ethyl-3-methylimidazole chloride) aqueous solution as solvent. Esterified starch and polylactic acid (PLA) were mixed to prepare composites. The structure and properties of esterified starch and its PLA composites were characterized by a Fourier transform infrared spectrometer (FTIR), a scanning electron microscope (SEM), and a contact angle goniometer. The results indicated that the optimal process for preparing esterified starch was reaction time of 2.5 h, reaction temperature of 85 ℃, catalyst dosage of 2% of starch, n(ionic liquid)∶n(water) of 1∶10, and n(esterification agent)∶n[dehydrated glucose unit (AUG) in starch] of 1∶3. Under the optimal reaction conditions, the degree of substitution was 0.92 mmol/g. FTIR analysis indicatesd that the starch has been successfully esterified. The contact angle test showed that the hydrophilicity of esterified starch decreased, similar to the contact angle of PLA. The DMA results showed that esterification modification improved the compatibility between starch and PLA. The mechanical property test results showed that when the mass fraction of esterified starch was 25%, the tensile strength, bending strength, and impact strength of the composites were increased by 4.7%, 132% and 26.5% compared to the PLA/ original starch composites, respectively. SEM results showed that the surface morphology of the esterified starch changed, and the compatibility between the esterified starch and PLA was improved.