In this article, the pretreatment before extrusion between the PLA and the modified starch was researched. The research on the composites of PLA and the modified starch focused on improving its compatibility and mechanical properties. The pretreatment samples were characterized by solid C-13-NMR and the chemical titration of the carboxyl end (-COOH) groups in PLA. The results illuminated that the pretreatment reaction was successful and the -COOH in PLA had reacted with the -OH in modified starch. The compatibility of the composites was determined by differential scanning calorimetry and scanning electron microscopy. The results showed that pretreatment could improve the compatibility of the composites. The mechanical properties of the composites were also enhanced. This approach is identified as a reasonable method to produce commercial PLA/modified starch composites with economical feasibility.