Aiming in reducing waste of natural resources, milled glass fiber (MGF), produced by grinding the residues from continuous glass fiber, is slowly coming into focus. To improve the interfacial interaction between the fiber and the renewable polymer PLA, silane coupling agents and SiO2 nanoparticles were used to modify the MGF surface via self-assembly strategy. Firstly, neat MGF (MGF-OH) and SiO2 were treated by the silane coupling agents, which changed the surface chemical properties; subsequently, MGF@SiO2 hybrid filler with nanoscale surface structures was prepared through mixing SiO2 bearing epoxy group with MGF bearing amine group via self-assembly, which changed the fiber surface topology. Here five different MGFs with different surface properties and two different size SiO2 were introduced into PLA matrix. The results show that MGF@SiO2, especially with 7-nanometer diameter SiO2, can accelerate the crystallization process of PLA better compared with MGF-OH and silane-treated MGF. Without the incorporation of SiO2 nanoparticles, the tensile strength of the composites, especially with MGF-OH loaded, shows a step down as compared to neat PLA. However, when the content of MGF achieved to 10 wt%, the tensile strength of PLA/MGF@SiO2 composite with 30-nanometer diameter SiO2 could increase to 83.4±2.9 MPa, meanwhile the value was improved by 20.7% compared with pure PLA. In addition, it is found that self-assembly is an effective way to help SiO2 disperse. The tensile and impact properties of PLA/MGF@SiO2 composites are better than those of the directly mixed PLA/MGF/SiO2 composites. The nanoscale interfacial structures formed by SiO2 and the reactive organic group from silane coupling agents have come together as the reasons for the enhancement of mechanical properties. We believe this simple and effective approach that using SiO2 nanoparticles as a novel fillers surface modifier may open a novel interface design strategy for developing high performance composites.