Boron carbide ceramics are often considered ideal materials for lightweight bulletproof armor, but their anomalous brittle failure at hypervelocity impact limits their use. Recent experiments have reported that nanotwins are ubiquitous in boron carbide and that nanotwinned samples are harder than the twin-free boron carbide, but although the strengthening effect of nanotwins on metals and alloys is well-established, their role in boron carbide ceramics is not well understood. In this study, we used classical molecular dynamics simulations to investigate how nanoscale twins affect the mechanical properties of boron carbide ceramics. Our classical molecular dynamics results show that introducing nanotwins in boron carbide can increase the shear strength limit by 19.72%, reduce the number of amorphized atoms, and narrow the width of the amorphous shear band. Under indentation load, nanotwins can also increase the compressive shear strength limit of boron carbide by 15.97% and change the crystal formation direction and region of the amorphous shear band. These findings suggest that twin boundaries can hinder the expansion of the amorphous shear band and provide a new design idea for improving the impact resistance of boron carbide ceramics and avoiding their abnormal brittle failure.