Abstract: From 19950701 in China a forced standard for engineering classification of rock masses(GB 5021894) is exectued This paper deals with main results concerning quantitative classification for basic quality of rock mass: criterion for classification,number of classes and mode of classification formula.Based on field and laboratory measured data of 9 variables from 103 engineering projects,a formula used to calculate BQ index of basic quality of rock masses was obtained through sufficient mathematical analysis,such as correlative analysis,cluster analysis,reliability analysis,stepwise regressive analysis and stepwise discriminative analysis.