Lightweight vehicle design has become an imperative in today's automotive industry. And it is a difficult task, which usually involves non-obvious decisions beyond the designer's intuition. In practice, optimization through finite element simulation is prohibitively inappropriate due to massive computational cost. As a consequence, approximation method is extensively used. In this paper, lightweight design of front side rail through high strength steel is performed. And the advantages of weighted average surrogate (WAS) for approximating the crashworthiness responses in frontal crash are also discussed. It shows the strategy of using WAS is effective, with great potential applications for vehicle crashworthiness approximation and lightweight design.