Abstract: To improve the interfacial compatibility of reed/poly(lactic acid) (PLA) composites, the reed fibers were surface-treated with citric acid (CA) in this study to introduce carboxyl groups and reduce the fiber polarity. The suitable ranges of the CA concentration, pH, and reaction temperature were determined through single-factor experiments, and the modification process was further optimized using an orthogonal experimental design. The results indicated that the optimal process conditions for CA modification of reed fibers were a CA concentration of 6%, a pH of 6, and a reaction temperature of 90℃. Under these conditions, the tensile strength of the composite reached 50.25 MPa, which was 49.2% higher than that of the unmodified group, and the elongation at break increased from 6.92% to 7.35%, demonstrating a significant enhancement in the mechanical properties. Scanning electron microscopy observations revealed that the tensile fracture surfaces exhibited close fiber embedment without noticeable fiber pull-out, indicating improved interfacial compatibility. The contact angle test results showed that the water contact angle of the composite increased from 83.77° to 94.87°, confirming a transition of the material surface from hydrophilic to hydrophobic. Thermal decomposition analysis further indicated that the end-set decomposition temperature of the composite was elevated, suggesting an improvement in the thermal stability. This study provides a new strategy for enhancing the interfacial compatibility of natural plant fiber-reinforced PLA composites.