The work aims to study the effect of different SiO2 contents and particle sizes on SiO2/PLA composite film. SiO2 was used as the additive to prepare silica/polylactic acid (SiO2/PLA) by tape casting method. The mechanical properties and barrier properties of the composite films were characterized by mechanical properties test, crystallinity test, heat stability test, oxygen permeability test and water vapor permeability test. Appropriate amount of SiO2 in PLA material enhanced the tensile strength and elastic modulus of composite films, and the smaller the particle size was, the more obvious the strengthening effect was. The mechanical properties of the composite films were better when the particle size was 50 nm and the content was 2%. At the same time, with the increase of SiO2 content, the oxygen permeability coefficient firstly decreased and then increased. When the SiO2 content was 2%, the oxygen permeability coefficient was the minimum and the oxygen barrier properties of the composite films were the best. The moisture permeability test results of SiO2 with different particle sizes and contents showed that the barrier properties of the composite films were the best when the SiO2 particle size was 50 nm and the content was 2%. In a word, the addition of SiO2 can improve the mechanical properties and barrier properties of PLA, and the difference in particle size and addition amount of silica has a great influence on the properties of the composite films.