Abstract: The unstable rock mass develop in the intersection section of Qinghai-Tibet Railway, G109 National Highway and Beijing-Tibet Expressway, which pose a great threat to the construction and safe operation of traffic engineering. The geological background, development characteristics and deformation failure mechanism of the unstable rock mass are systematically analyzed in order to find out the instability conditions, the main controlling factors and to evaluate the stability. Using block theory and discrete element method, the stability and dynamic response of dangerous rock mass under dead weight condition and seismic load are studied respectively. The study shows that the unstable rock mass can be divided into three parts: the integral collapse source at the top, the fissure zone in the middle and the accumulation area of the block stone in the lower. and there are four groups of dominant structural planes. The calculation results show that the dangerous rock block is basically stable under the condition of dead weight. The unstable modes of dangerous rock mass under the action of seismic load are shear sliding, fracture and fall. Short-time violent earthquakes intensified the accumulation of internal damage in the unstable rock mass. To ensure safety, it is suggested to carry out targeted prevention and control, monitoring and early warning.