The work aims to improve the poor interfacial bonding between the reinforcement phase and the matrix phase of fiber reinforced polylactic acid (PLA) composites. The compatible interface of SP/PLA composite was constructed with straw powder (SP) as filler and nano titanium dioxide (TiO2) as interface modifier. The effects of different content of nano titanium dioxide on the mechanical properties and interface compatibility of SP/PLA composite were investigated by means of mechanical property test, water absorption test, scanning electron microscope (SEM), X-ray diffraction (XRD) and thermogravimetric analysis (TGA). The study showed that the tensile strength and bending strength of the composite reached 42.78 MPa and 91.25 MPa respectively, and its water resistance, crystallinity and heat resistance were also the best when the content of nano titanium dioxide was 2.0%. Nanometer titanium dioxide can effectively improve the properties of straw/PLA composites.