At conceptual design stage, beam element is extensively used to create the frame structure of automotive body, which can not only archive the accurate stiffness but also reduce much design period. However, so far there is no perfect method to apply the beam element to create the automotive frame composed of the plate element. This paper presents a solution to this problem in order to help engineers to fast carry out the vehicle body problem at conceptual design stage. Firstly, formulations of geometric properties of complex section are reviewed. Secondly, the method of establishing the cross beam with reference to the midpoint deflection and mass of the plate is presented to simplify the plate with a higher precision. Thirdly, regarding the joint elements of vehicle body, the spatial semi-rigid beam element and its stiffness matrix are expressed. Lastly, a numerical example of car frame proves that the proposed method can analyze the stiffness of the body more fast and accurately.