The evaluation of brittleness of tight sandstone gas reservoir is of great significance in guiding fracturing construction. In view of the problems that the brittle minerals could not be determined in the evaluation of the brittleness and ignorance of influence of diagenesis on the rock brittleness，first of all，the relationship between the mineral components and the brittle index was systematically analyzed according to information of thin sections，core data and the triaxial compressive stress under formation condition based on 26 samples from the block in the northeastern Ordos Basin. The quartz and feldspar are the main brittle minerals in the study area. Secondly，based on two different correlations of brittle index with the main brittle minerals，the influence of diagenesis，pore size and brittle mineral difference on the brittle index was determined. And then the influence of the diagenetic minerals on the pore size and the degree of diagenesis was synthesized to classify tight sandstone reservoirs into two types ofⅠreservoir andⅡreservoir by using ratio of quartz and debris to feldspar content. Finally，the brittleness evaluation method for tight sandstone reservoirs based on mineral compositions and diagenesis was proposed and the corresponding brittle index evaluation model was established. The results show that the brittle index evaluation model based on mineral compositions and diagenesis can obtain more accurate brittle index of tight sandstone reservoir，which can provide parameter basis for the identification and stimulation of high-brittle fracturing target layer.