The B pillar is one of the key load-bearing structural members of vehicles. To improve the side crash performances of a vehicle and realize the lightweight design of the B pillar, a hybrid structure of B pillar made from carbon fiber reinforced composite (CFRP)-tailor rolling steel blank (TRB) was adopted based on structural and different material characteristics of the B pillar. The traditional reinforcing plate in the middle of the B pillar was removed, and the B pillar outer plate was designed as a TRB steel plate with the CFRP partially attached to the inner side of the B pillar outer plate for reinforcement. In addition, the thickness distribution of the inner and outer plates of the hybrid structure B pillar and the layup order and layup thickness of the reinforced composite were optimized based on the software of Optistruct. Finally, the side crash simulation results of the whole vehicle show that the optimized hybrid-structure B pillar achieves a weight reduction of 27.7% and the resistance to side impact is improved.