The work aims to research the effect of nano-SiO2 on the crystallization behavior and mechanical properties of biodegradable poly(3-hydroxybutyrate-co-4-hydroxybutyrate) (P34HB) packaging film. SiO2/P34HB nanocomposite films were prepared by solution casting method. The effects of nano-SiO2 on the structure, crystallinity and mechanical properties of P34HB were analyzed by FTIR, SEM, POM, DSC and mechanical universal testing machine. The results in-dicated that, nano-SiO2 played a heterogeneous nucleation role in P34HB. The crystallization rate and crystallinity of SiO2/P34HB composite films were significantly improved. Compared with P34HB packaging film, the elastic modulus and tensile strength of SiO2/P34HB composite film were increased respectively by 72.7% and 60.9%, when the mass fraction of nano-SiO2 was 2%. Finally, the optimum doping ratio parameters of nano-SiO2 for improving the crystallinity and mechanical properties of biodegradable poly(3-hydroxybutyrate-co-4-hydroxybutyrate) packaging film were obtained.