To further understand the plastic deformation in lath martensite, a molecular dynamics simulation in combination with experimental measurements is carried out. The models with different misorientation angles, twist components, and pre-screw dislocation are created to reproduce the complex structure of martensite lath boundaries. The tensile experiment is conducted by using low-carbon steel. By combing simulation results, the mechanism of slip band formation in lath martensite is quantitatively analyzed.