This study examines the interface of biodegradable polylactic acid (PLA) fiber-reinforced thermoplastic starch composites to address market demand for their mechanical properties. We first used a combination of solution treatment (acid/alkali) and plasma modification to modify fibers. This method improves surface roughness and increases oxygen-containing groups of the fibers, which enhances the mechanical interlocking at the interface and promotes stronger hydrogen bonds between fibers and starch, respectively. Their synergy effect significantly strengthens interfacial adhesion and interfacial shear strength (IFSS). In contrast, cold plasma modification alone results in a smaller increase in IFSS because of its lower roughness and fewer oxygen-containing groups. Alkaline treatment and increased cold plasma power level are more conducive to higher IFSS through stronger synergistic effects. Although IFSS improved from 2.41 to 4.40 MPa after alkali and plasma treatments of 200 W, the tensile strength decreased from 811.46 to 351.55 MPa. To optimize the mechanical properties of composite materials, it is crucial to choose a kind of modification method that balances IFSS enhancement and tensile strength reduction.