The construction of a highway in mountain areas would influence the environment along the line, in the form of the water environment pollution, the air environmental pollution, the sound pollution,the ecological environment pollution and the social environmental impact. Therefore, the assessment of the highway construction environmental damage is of great importance. In order to evaluate the overall impacts of the area's highway construction project on the district environment, this paper proposes an indicator system of the environmental impact assessment, and uses the extension theory to evaluate the impairment of the environment by the matter element extension method. As an example, the Zhangjiajie-Huayuan expressway construction project is evaluated. The evaluation matter element model of the environment effect is established according to the survey data collected on the Zhangjiajie to Huayuan expressway. At the same time, the extensional evaluation method with correlative functions is used. It is shown that the evaluation results obtained from this method by eliminating the human intervention, using the correlated function and the weight coefficient of the element extension method agree well with the real situations, and represent the main characteristics of the environment influences.