Abstract: This paper presents the key technology on the lightweight design of allplastic car bodies.According to the characteristics of rotational molding, the mold design, and process requirements，the structure of allplastic car bodies was designed reasonably, which met the overall appearance in order that the structure matched with the function well.They were mainly reflected in the combination of the whole car body, the lampshade, the fender,the hood, and other key parts in order to achieve the integration and lightening on the structure of design.