This paper presents the lightweight design of one kind of composite materials in automobile body-in-white. Front longitu- dinal beam reinforcement of a domestic senior c-classsedan is taken as the research object, steel and composite reinforcement are studied and compared with both on the influence of automobile crash safety and modal as well as stiffness and other performances. The analysis and comparison of the results show that the composite front longitudinal beam reinforcement can meet the key performance requirements of the car. The research above indicates the composite reinforcement can well replace of the steel reinforcement while its weight loss of 3. 94kg and the ratio of weight loss reaches up to 53. 4%, which provides vital guidance and reference significance for the application of composite materials in the lightweight design of automobile body.