Taking the glutenite formation in tight gas reservoir of Songbei area as an example，the brittleness characteristics of glutenite containing different gravel size are studied by performing tri-axial compression tests. Based on the theory of energy evolution，the influence of gravel size on the mechanical properties and brittleness of glutenite is analyzed according to the investigation of the characteristics of broken rock and deviator stress-strain curve. Additionally，the acoustic emission response during the rock being broken process is analyzed. The experimental results of tri-axial compression test show that gravel size has a significant influence on the characteristics of broken rock samples. Compared with small size（<1 mm）gravel-dominated rock samples，large size（2-6 mm）gravel-dominated glutenite samples are sheared and broken at lower confining pressures with a smaller dip angle of fracture plane. Because of the existence of a large number of weak cemented gravel interfaces，the rock samples with larger size gravel are usually of lower compressive strength，while the broken pattern is more complex. In addition，the unevenness of the fracture surface leads to a part of the energy dissipated by the friction of the fracture surface during the failure process of the rock sample. Consequently，the brittleness of the gravel with larger gravels is lower. Moreover，the interpretation results of acoustic emission show that the proportion of shear events is higher in small size gravel-dominated rock samples，indicating shear failure tended to be dominant.