According to the statistics, among large area blackouts in power system, there are more than 70% of the accidents are linked to the secondary system directly or indirectly. So the reliability of the secondary system is vital to the establishment of a strong and health grid. Considering the risk of secondary system affects the reliability of the power system, this article makes a research with application of the MCMC method and puts forward 4 reliability indices of the grid as the measure of the secondary system risk. The MCMC method inherits the advantages of traditional MC method, which has a fast convergence speed, accurate calculation and stable index. Meanwhile the MCMC method realizes dynamic simulation. Through the simulation of IEEE RTS - 24 nodes, this article illustrates the validity and stability of the MCMC method. Finally, taking a Guizhou local grid as an example, this paper calculates 4 reliability indices under 4 typical operation modes. The results show that the secondary system risk is larger in winter operation mode, which has great reference value for establishing a more reliable and security power system.