The improved slope mass rating method(SMR) is proposed, through integrating the gray system theory with traditional SMR method. The quantitative indexes value of traditional rock mass classification methods was very discrete, resulting in the step change in classification. The problems of small samples and discrete issues are solved by using features of gray system theory. Firstly, the evaluation indexes of traditional rock mass quality classification are classified. Also, the weights of indexes are determined; and the triangle whiten function for evaluation indexes is built. Furthermore, the rock mass quality classification of slopes is valued based on the maximum degree. Finally, combining with a slope case study and comparing general engineering rock mass classification methods(RMR and SMR), the result judged by improved SMR method can agree well with engineering facts; and it expresses a high and strong quality grade stability. For rock mass quality classification in slope engineering, this study shows that this method is scientific and accurate.