Based on the analysis of energy-consuming characteristic of mechanic products, the abstract process model of energy consumption was established from the viewpoint of product design, and the energy-consuming model for life cycle is deduced for the material, structure, and manufacture system. For each stage of product life cycle, the relationship between energy factors and energy consumption was analyzed by adopting the theory of bond graph. And then the quantitative energy formula for each stage of life cycle was presented. The design scheme optimization was carried out by the expanded application of the model. The model and method in the paper were validated by a product case.