Bi-Directional Interpolation Model (BDIM) decided by a threshold is presented in this study to treat intermediate food vehicle frame density elements. Based on BDIM a mathematical model of food vehicle frame topology optimizing is established by integrating the analysis of the traditional interpolation model, only acting on sensitivity with BDIM and acting on sensitivity and rigidity with BDIM under the Gradient Projection Method. And it is exposed that the only acting on sensitivity with BDIM can get a global convergence and boundaries clear optimization results, which provides a new method to resolve the difficult problem of intermediate density elements. This study discusses its application in the field of food transportation vehicle frame.