A novel TPA method was proposed to study the product packaging coupling system based on the spectral-based substructuring technique. The new method was applied to study the component level transfer function which made the largest contribution to the target system transfer function, and to find out the influencing law of the coupling interface characteristic parameters on the force transfer rate, so as to better guide the design of cushioning packaging. A two-substructure multi-coordinate coupled system lumped parameter model was used to study the component level transfer function and the contributions of the various coupling points to the target system transfer function. Also the influencing law of the special physical parameters at the coupling interface on the force transfer rate was studied. As a result, the component level transfer function with the largest contribution to the target system transfer function was found out, as well as the influencing law of the stiffness and damping at the coupling interface on the force transfer rate. In conclusion, the novel method was proved to be efficient and feasible to be applied in path contribution analysis, and it could efficiently recognize the contributions of the sub-structuring transfer pathways to the peak value of the target system transfer function, as well as reveal the influence of the physical parameters at the packaging coupling interface on the transfer contribution. And it provides much theoretical support and technical guidance for the design and optimization of cushioning packaging.