Carbon fiber composites have the characteristics of light weight and high strength, and are widely used in automotive cover parts. This paper targets at the B-pillar reinforcement plate of the passenger compartment, and proposes a lightweight-oriented design of carbon fiber composite B-pillar reinforcement plate through topology optimization, structural design and initial layup design. The stiffness and three-point bending of the carbon fiber composite B-pillar reinforcement plate are compared, and its crashworthiness index is calibrated. Based on the side impact analysis of the whole vehicle model, the peak intrusion velocity of the carbon fiber composite B-pillar reinforcement plate is lower than that of the original steel material B-pillar reinforcement plate, and the intrusion velocities of the corresponding dummy's thorax, abdomen, and pelvis are reduced by 0.8 m/s, 0.5 m/s, and 0.5 m/s, and the intrusion amount is reduced by 8 mm, 14 mm, and 16 mm, respectively, compared with that of the original steel material B-pillar. The study shows that the carbon fiber composite B-pillar reinforcement plate can effectively improve the crashworthiness of the whole vehicle, and it is of value to be promoted in the study of carbon fiber composites applied to other structural parts of the body frame based on lightweight design.