On the basis of constitutive relationships of expanded polystyrene (EPS) and C-flute corrugated paperboard (CCP), the cushioning mathematical model of goods and exterior and interior packaging composed by the two kinds of material was established. Optimal design was carried out on engineering project and application of the series model was introduced. The results showed the series cushioning effect of exterior packaging should not be neglected in engineering design; otherwise, over packaging will happen.