The paper aims to study the effect of modified silica on mechanical properties, oxygen permeability and water vapor permeability of polylactic acid. In this study, industrial grade silica with a particle size of 50 nm was selected as an additive to be modified with KH570 silane coupling agent; and then the modified silica was blended with polylactic acid by solution casting to prepare a film. The tensile properties test, oxygen permeability test and water vapor permeability test analysis were used to characterize the mechanical properties and barrier properties of the composite film. The results showed that, compared with the pure PLA film, the tensile strength and elastic modulus of the modified composite film were increased by 18.65% and 19.91%, respectively; the glass transition temperature was about 11 °C higher than that of the pure PLA film; and the thermal stability was enhanced. Compared with the pure PLA film, the oxygen permeability coefficient and the water vapor permeability coefficient of the modified composite film were reduced by 29.89% and 43.76%, respectively, and the barrier property was significantly improved. The silica modified with KH570 silane coupling agent has better effect on improving the performance of polylactic acid materials, providing a theoretical basis for the application of polylactic acid materials in packaging.