Physical model of unstable sliding rock and dynamic calculation model for unstable sliding rock excavation blasting are established by the peak velocity decay rule of blasting and the mechanical characteristics of unstable rock mass. Blasting dynamic stress from any position in the unstable rock can be confirmed by the Euler equation, and a simplified method is proposed to obtain the blasting equivalent load and to correct the equivalent load. Combining with ultimate equilibrium method, it will attain coefficient of stability from unstable rock at any moment. In the end, the dynamic calculation method of unstable sliding rock is constructed. Taking Wangxia unstable rock W2-1 from Chongqing as an example, the time history curve of unstable rock with stability coefficient can be drawn. The dynamic effect of blasting caused the stability coefficient to be reduced by 2.86%, and aggravated the danger of unstable rock. Action time of blast power to the unstable rock is about 0.45s, which is coincident with the data of field monitoring. It can provide the theoretical foundation to assess the calculation of unstable rock stability and to optimize the design of excavation blasting.