Achieving sustainable development while limiting environmental pollution is one of the main enormous challenges for chemical engineering at present. To develop and design a greener alternative to replace or retrofit a current polluted process, it is essential to establish a method for quantitatively evaluating the environmental impact of a chemical process so that its environmental performance can be improved by identifying and discovering the bottlenecks that cause the pollution. In this work, a green degree (GD) method is proposed to quantitatively evaluate the environmental impact of a chemical process and related energy-generation system. Definitions and calculation formulas of green degrees for a substance, a mixture, a stream, and a unit (process) are illustrated. The green degree is an integrated index that includes nine environmental impact categories (including global, air, water, and toxicological effects). Therefore, it is comprehensive for assessing and understanding the environmental performance of a complex multicomponent chemical system. Three illustrative case studies are presented to further describe and verify the applicability of the method: (I) solvent screening by comparing the green degree values of solvents, (2) process route screening for producing methyl methacrylate (MMA), and (3) green degree analysis of the inethyl methacrylate process (i-C4 route) by integration with process simulation technology.