Abstract: Metallic glasses exhibit promising potential in many engineering applications due to their attractive mechanical properties. Compared to their crystalline counterparts, however, the metallic glasses appear to be inherently brittle. To overcome this limitation, introducing the nanocrystals into the amorphous structure can improve significantly the plasticity of metallic glasses. Nanocrystals were formed in situ into the Pt-based bulk metallic glass (Pt-BMG) matrix using the thermal induction method of annealing. The mechanical properties and plastic dynamics behavior of the as-cast and glass transition temperature T_g (250 °C) annealed for 15 min, 2 h, and 6 h were investigated by nanoindentation experiments. The results show that when the annealing time increases from 15 min to 6 h, the crystallinity of Pt-BMG increases from 34% to 57%, the average grain size increases from 25.6 nm to 38.3 nm, the hardness and reduced modulus increase from 5.66 GPa and 133.83 GPa to 8.65 GPa and 182.89 GPa, respectively. Meanwhile, the serrated flow behaviors on the load-displacement curves show a pattern of obvious discontinuous displacement abrupt to relatively smooth changes. Through molecular dynamics simulations, it has been further demonstrated that as the size of the nanocrystals increases, the activation of the shear transition zone and the nucleation of the shear band exhibit a trend of first promoting, then inhibiting, and first increasing and then decreasing. This is because during the plastic deformation process of bulk metallic glass, small-sized nanocrystals are wrapped or dissolved by shear bands, promoting the formation of plastic deformation of bulk metallic glass; However, large-sized nanocrystals generate dislocations and slip within the crystal when subjected to loads, further suppressing the nucleation and propagation of shear bands. By means of nanoindentation experiment and molecular dynamics simulation, the internal mechanism of nanocrystalline size affecting the plastic deformation of amorphous alloys is revealed from the atomic scale, which provides an effective experimental basis and theoretical support for the design of metallic glasses with ideal properties.