Environmental impact assessment of design and manufacturing system decisions have attracted significant attention in the recent years. Researchers have focused on the effect of product, process design decision on the environment during the whole product life cycle. Establishing a quantitative and effective assessment model to assess the important degree of environmental impacts became more and more important. This research analyzes the characters of EIA, describes the hierarchy of environmental impacts assessment and sets up the calculated equation. Fuzzy AHP and Entropic Coefficient method are applied to determine the synthesis weight of the various evaluation indexes. And the sequence of important degree of environmental impacts, which is obtained in the model, can also direct the companies' managers, process designers and product designers to improve environmental impact with finite resources. At last, the model is demonstrated by a case study.