Abstract: The strength ratio is an important factor affecting the stability of the surrounding rock during tunnel excavation of deeply-situated composite rock mass. A size-dependent bond contact model is implemented to the software of the two-dimensional distinct element method (DEM) to simulate the tunnel excavation of deeply-situated up-soft/low-hard composite rock mass. The influences of strength ratio on bond breakage, the maximum principal stress and disturbance zone of the surrounding rock are investigated. The results show that the bond breakage ratio and the disturbed area ratio caused by tunnel excavation gradually increase with the increase of strength ratio of composite rock mass, and the bond breakage is mainly caused by bond tensile failure. The maximum principal stress is distributed in teardrop shape in the circumferential direction of the surrounding rock, while in the radial direction it decreases firstly and then increases in the soft rock and increases in the hard rock. Moreover, in the radial direction, with the increase of the strength ratio, the variation range of the maximum principal stress decreases in the hard rock but increases in the soft rock.