Abstract: Based on studies on the characteristics of membership function,it was indicated that its principal interval could be divided into wide domain model and narrow domain model and that there were three types of its transitional zone forms,namely mutation,linear and nonlinear,and there were the clearest point and the most fuzzy point among the reference point of the function graph.According to the characteristics of membership function and the theory of the summation of membership degree equal to 1,the constructing method of integral fuzzy membership function and the calculation method of the parameters were brought out.Based on rock mechanics and the theory of fuzzy synthetical assessment and taking the fuzzy synthetical classification of rock stability as an example,it was proved that although there were differences among concrete values of fuzzy membership,the final classifications were consistent when the membership functions were constructed by different basic functions according to the above method.As an application of this constructing method of integral fuzzy membership function,the fuzzy classification system of rock construction site of highways in southwest mountainous region was established.Some geomechanical conditions,such as uniaxial compressive strength,softening coefficient because of absorbing water,integrated modulus and karst development degree,and some geographical conditions,such as the mountain land type,slope land type,were taken into account in the system.Finally,this system was accepted by the design and construction engineers,and was used to classify a construction site of highways in mountainous area.