In order to improve the stiffness and reduce the cost, the stiffness analysis of the flat vierendeel structure was taken in to account with the analytical method and FEM. The structural main geometrical parameters’ influences on stiffness were studied, such as the height of the structure and the area of the upper and lower chords. The study showed that there was a most reasonable height for any flat vierendeel structure, which could make the stiffness best and the deformation least, and the formula for the reasonable height was deduced. Furthermore the area of the chord’s influence on the most reasonable height was discussed. At last, case study by FEM proved the validity of the proposed method, and several measurements for stiffness optimization were also put forward, which could be a reference for the design of relevant projects.