B-pillar and rocker are the key force transmission sub-systems of the side impact of battery electric vehicle (BEV), and scholars have studied the side crashworthiness of these sub-systems and vehicle body a lot. However, these works are insufficient on the analysis of benchmarking vehicle models, the simulation and experiment of the B-pillar and rocker sub-systems, and the optimization of these sub-systems. To make up these shortcomings, this work aims to design the B-pillar and the rocker and improve the side crashworthiness of BEV. It presents a systematic method on the side crashworthiness of BEV. The dynamic bending performance of the B-pillar and the rocker are studied in simulation analyses and experiments. The materials, structures, and crashworthiness of these two sub-systems of eleven benchmark models are studied. Minimizing structural mass and cost as well as maximizing mean crushing force are performed based on multi-objective artificial tree algorithm to optimize the B-pillar and the rocker. The optimized sub-systems are applied to the body of a BEV. As a result, the performance of the B-pillar and the rocker is significantly improved, and their optimal solution templates are provided. The performance of the BEV is also improved under the advanced European mobile deformable barrier side impact and the oblique pole side impact. Some interesting conclusions for BEV are presented. In summary, this work has obvious reference value for automotive engineers and scholars to further study the crashworthiness and lightweight of BEV.