Atomistic details of the interaction between screw dislocation and substitutional solute Si in Fe-Si alloy was investigated by atomistic modeling method. We developed an embedded atom method (EAM) potential for Fe-Si interaction based on the density functional theory calculations and then evaluated the interaction energy between Si and screw dislocation using the developed potential. The interaction energy is found to become larger as the Si atom approaches the screw dislocation. This indicates that attractive driving force acting between the Si atom and screw dislocation can lead to the athermal resistance. Using Nudged Elastic Band (NEB) method, we computed the energy barrier for screw dislocation glide associated with double kink formation, and found that the energy barrier is reduced by nearby existing Si atom. These results suggest that solute Si causes both solid-solution hardening and softening in Fe-Si alloys.