Abstract: Environment impact (EI) is a multi- decision-making, multi-objective and multi-criteria system with uncertainties and ambiguities, and environment impact index (EII) of a chemical compound is difficult to calculate appropriately with traditional methods.In this paper, fuzzy optimal selection theory based on the concept of relative importance degree is adopted to determine relativity of the degree of environment impact of chemical compounds as EII, and environment impact assessment (EIA) of a chemical process is done with values of compounds EII to select alternatives that cause less pollution to the environment.It is useful in further process optimization.The paper presents a practical application to the treatment of process condensate in a synthesis ammonia plant to illustrate the procedure, and the calculation result demonstrates the feasibility, practicability, and effectiveness of the suggested method.