Since the study of the brittleness anisotropy for shale rocks is relatively rare, it is necessary to study the corresponding brittleness characteristics of the shale reservoir. Therefore, the authors construct an effective medium model to link elastic properties and brittleness index of shales to complex constituents and specific microstructures. Based on upscaling modeling, the authors sequently construct the nano-porous clay composite, submicron organic-rich clay composite and micron dry rock matrix by introducing clay particles. In consideration of the factors such as bound water and adsorbed gas in nanopores, the authors construct an effective medium model of shale reservoirs to characterize the elastic parameters, which improves the construction process based on DEM and Gassmann theory. Then, the anisotropic expressions of Young's module and Poisson's ratio based on elastic modulus parameters are derived by using the transition matrixes M and Bond conversion. Finally, the anisotropy features of shale rock's brittleness and the seismic response are summarized with different mineral constituents. Brittleness analysis shows that the brittleness in the direction that is parallel to the strike and perpendicular to the bedding is 2.5 times that perpendicular to strike and inclined to bedding at a 45 degree and the rock brittleness has increased, which suggests that the rock is more likely to fracture with increasing quartz content and decreasing clay content. These results provide a reference for predicting reservoirs' brittleness and designing hydraulic fracturing.