Geographical and geologic elements that influence the constructi on environmenton highway engineering in mountainous area were analyzed, and an issue on environment grade for construction of highway engineering was put forward. Five key elements, namely the development rate of Karst, acquiring rate of integrated rock-mass core from bore, characteristics of softening for rock-soil, terrain and gradient, were pick out and considered as the indexes of construction environment classification. The expressions of subjective function for various elements were established based on the method for segment construction as well as the equivalent principle of subjective function. A new objective calculation method of fuzzy weight that various influencing element indexes subjected to construction environment classification grade was proposed based on the knowledge dependent degree and the importance axiom in rough set theory. Fuzzy evaluation method of construction environment grade of highway engineering in mountainous area was built through the classification index combining with subjective function and calculation method for fuzzy weight. An engineering example was analyzed by the fuzzy evaluation method of construction environment grade. The analyzed result was regarded as an important data when making decisions for engineering construction. Validity and practicability was revealed through above-mentioned practical example.