A toughener and glass fiber (GF) modified polyethylene terephthalate (PET) material was used to study the vehicle body design. Considering the material’s high impact toughness, high specific strength, high specific stiffness and the characteristics of the injection molding process, the non-loaded body adopts a block design and is connected to the body by structural glue, which can minimize the weight of the vehicle body and realizes the weight reduction of the whole vehicle. The experiment shows that the modulus and strength difference of the 15% short glass fiber reinforced PET can reach 40% ~50% because of glass fiber orientation, thus makes it difficult to obtain accurate simulation results for guiding the design. In this research, the microscopic and basic mechanical properties of the glass fiber reinforced PET composites were studied. The high precision spline simulation was established with engineering software such as Moldflow, Digimat and Abaqus. The material parameters were corrected and the results were used for stiffness and modal simulation of the whole injection molded vehicle. The simulation results were compared with the test results, and the quasi-static simulation accuracy of the injection molded product was improved to over 85%.