Recent experiments have revealed that external bending breaks the symmetry of lithiation in germanium nanowires. However, the effects of external stress on lithiation in silicon and the associated underlying mechanisms remain unclear. Here, we have performed a series of large-scale atomistic simulations based on a newly developed reactive force field (ReaxFF) to investigate the effects of external stress on the interfacial reactions and diffusion (two dominant processes during lithiation) of silicon anodes. The simulation results quantitatively show the variations in the migration velocity of the phase boundary (i.e., the reaction front during lithiation) and the diffusivity of lithium (in crystalline and amorphous lithiated silicon) as a function of the external stress and indicate that the tensile stress accelerates the lithiation rate while the compressive stress retards it. Furthermore, comparing the large-scale ReaxFF-based simulations and previous ab initio molecular dynamics (AIMD) simulations indicated that large samples and long duration times are crucially important and indispensable for accurately calculating the diffusivity of amorphous lithiated silicon under external stress. These results not only provide a fundamental understanding of the intimate coupling between mechanical stress and lithiation kinetics but also open avenues for optimizing batteries to control/alter the lithiation rate of silicon anodes by applying external mechanical stresses.